In "48 Hours" they destroy the last functioning DHD on Earth in order to save Teal'c, however at the same time they establish at least a rudimentary knowledge about how to hook up and unhook a DHD device. It has also been proven in many other times that they can dial the gate without the DHD.
What stopped them from going to some uninhabited back water world, unhooking the DHD there and bringing it to Earth so that they could have a fully functional DHD?
The SGC in the early days before the Prometheus, they tried to avoid cutting off worlds from the gate network. Removing a DHD could potentially trap someone who arrived on the world and with no way to effectively survey a world beyond 10 to 15 miles from the gate it wasn't worth the risk.
As far as gates go, until Prometheus they didn't have a reliable means of interstellar travel that they didn't have to negotiate for. And I would assume that the Jaffa Rebels and the Tok'ra would both not approve of loaning the SGC a ship to break a part of the gate network. Everyone seems to be a bit pissed when Anubis does it.
By the time they could gather DHD's they had sufficient knowledge of the gate control systems that they no longer needed one, and their own "MacGyvered" had more precise controls and was resistant to DHD system failures like Baal's DHD virus. And by the time they built the McKay-Carter Intergalactic Bridge they had such mastery of the gates no longer needed DHDs.
Early in the SG-1 era, it would have been difficult, as you need a ship to transport the DHD, and until the Prometheus Earth didn't have one. Once they got the Prometheus and the later, more advanced ships, it became feasible. However, any time they needed an extra DHD, it was an emergency, so they didn't have time to grab one from a nearby system anyway.
In the late-SG-1/mid-Atlantis era, they should have had a ton of extra DHDs lying around. After the McKay-Carter Intergalactic Bridge was built, they should have had a maximum of 17 Milky Way DHDs and a maximum of 17 Pegasus Galaxy DHDs sitting in storage somewhere.
A good reason they might have stuck with the dialing computer over a DHD could have been security. The computer allowed for other things, such as biometric security.
Maybe SGC was afraid that it will somewhat break the status quo achieved with Russians. Of course they could just not tell them that they brought the DHD, but I believe Russians would sooner or later know and be angry, because that would mean another advantage of USA Stargate program. Russians could demand restoration of their wormhole travels as a "recompense" or something like that.
I think they certainly could have once they had the Prometheus, but honestly I don't think it would be worth the hassle, because while they did have a need for a second gate several times, they never needed a secondary way to dial their gate.
I also agree with some of the others who answered this first, in that an Earth-made DHD was better than the normal DHDs because of the added security, and also no one else could try to hack into Earth's homemade DHD because they did not have another one to reverse engineer, which made them impervios to Ba'al's gate virus.
It's worth noting that after twelve years or so, when the SGC has plentiful knowledge about how to interface with a Stargate and about how to properly implement a dialling computer, when given the chance to start from scratch on Icarus they did blend a dialling computer with a DHD. Presumably, if the SGC didn't already have a fully functional integrated system itself, this would have been a viable option by that point.
They should already have a DHD from Antarctica. They never explained what happened to that device after they recovered the gate itself. Additionally in Atlantis they salvaged unused gates to build a space bridge. But what did they do with those DHDs?
Come to think about it, since they managed to go salvage gates for this gate bridge, why continue paying rent to the Russians for the gate that they have. If it were up to me, they could just give the thing back and get another one. It made sense at the time they started renting the gate because they did not have any ships, but later the Russians wanted to take their gate back and the SGC was forced to hand over one of their ships to buy the Russians out of the contract.
Didn't the Gate need to have a specific configuration to be able to be used as a starting point?
In first movie they said that Abydos gate had a completely different configuration of symbols. So if you took Abydos gate to Earth you would not be able to start sequence, because of lack of Earth's seventh symbol?
Not saying that you could not use it as an end point i.e. any gate at set coordinates would be able to accept incoming wormhole.
But you could not set the Earth's seventh symbol as point of origin on some gates.
So maybe the choice of which gate would be usable on Earth is quite limited.
Battery was completely drained with no (known) way to recharge it.
Definitely could have/should have, especially in the earlier years of the program. Didn't, probably because of worries of stranding people, but that fails the intelligence test (just "bury the gate" after they take the DHD). Maybe didn't because they weren't sure they could get it to work with our gate, but still should have been tried/mentioned in the series.
Continued "renting" due to political pressure AND the need for secrecy (Russian's threatened repeatedly to reveal the program to the world). So it was politically expedient to pay to keep the Russians quiet and the rentals provided a pseudo-legitimate way to do that.
A stripped down MALP would have been more than sufficient to carry a DHD back through an open Stargate, which doesn't require a DHD to maintain an open wormhole (see also: "manual dialing"). Alternatively, they could have taken the Antarctic DHD and swapped in a replacement power crystal from any of the many, many DHDs where the interface was blasted, but not the power crystal.
Out of universe: why spend the money to re-vamp their primary stage setting when what they had was working great already?
