Mrs Weasley stopped him from building up his knowledge to any great extent.
Arthur Weasley was an enthusiast for all things Muggle. But his expertise was hindered by a rather suspicious and judgemental wife, who didn't care much for his experiments with Muggle technology. Arthur took to hiding these experiments from his wife.
"C-cars, Molly, dear?"
"Yes, Arthur, cars," said Mrs Weasley, her eyes flashing. "Imagine a wizard buying a rusty old car and telling his wife all he wanted to do with it was take it apart to see how it worked while really he was enchanting it to make it fly."
Mr Weasley blinked.
"Well, dear, I think you'll find that he would be quite within the law to do that, even if, er, he maybe would have done better to, um, tell his wife the truth...There's a loophole in the law, you'll find...as long as he wasn't intending to fly the car, the fact that the car could fly wouldn't- "
"Arthur Weasley, you made sure there was a loophole when you wrote that law!" shouted Mrs Weasley. "Just so you could carry on tinkering with all that Muggle rubbish in your shed!"
(Chamber of Secrets, Chapter 3, The Burrow).
"There's no need to, er, mention it to Molly," Mr Weasley told Harry, blocking his access to the coop, "but, er, Ted Tonks sent me most of what was left of Sirius's bike and, er, I'm hiding - that is to say, keeping - it in here. Fantastic stuff: there's an exhaust gaskin, as I believe it's called, the most magnificent battery, and it'll be a great opportunity to find out how brakes work. I'm going to try and put it all back together again when Molly's not - I mean, when I've got time."
(Deathly Hallows, Chapter 6, The Ghoul in Pyjamas).
Since his wife was not the most...supportive of people Arthur had to conduct his experiments on Muggle tech in secrecy and on his own. He could only do this occasionally whenever he had some spare time.
Arthur is a pureblood. He didn't grow up in the Muggle world and both of his parents were magical. He's had to build his knowledge of the Muggle world up from scratch. Imagine if you found a piece of alien tech and had to work out how it operated with no manual or frame of reference. Arthur learned the best he could in difficult conditions.
Remember as well that Arthur may love the Muggle world but he still spends most of his time in the wizarding world. He goes into the Muggle world temporarily in order to solve problems but this contact is only fleeting and necessarily involves him going straight back to the wizarding world to fill out the paperwork. Any Muggles he talks to have to have their memories wiped immediately. His scope for learning is really limited.
His best hope of learning more about the Muggle world is to talk to Muggleborn witches and wizards. When Harry comes to stay he grasps this opportunity with both hands.
Mr Weasley liked Harry to sit next to him at the dinner table so that he could bombard him with questions about life with Muggles, asking him to explain how things like plugs and the postal service worked.
"Fascinating!" he would say, as Harry talked him through using a telephone. "Ingenious, really, how many ways Muggles have found of getting along without magic."
(Chamber of Secrets, Chapter 4, At Flourish and Blotts).
Again, these sorts of conversations were probably few and far between. Perhaps older Muggleborns are more coy about their upbringing. Or maybe Mr Weasley doesn't know who's a Muggleborn and who isn't. But since Mrs Weasley regards her husband's fascination with all things Muggle as a acute source of social embarrassment it's a fair bet that he doesn't often get the chance to quiz anyone who really knows about the Muggle world.
That said Mr Weasley is actually pretty knowledgeable...for a wizard.
Mr Weasley isn't actually a total dunce. He does have some awareness of how Muggles operate. Consider when he goes camping.
"Right," he said excitedly, "no magic allowed, strictly speaking, not when we're out in these numbers on Muggle land. We'll be putting these tents up by hand! Shouldn't be too difficult...Muggles do it all the time..."
(Goblet of Fire, Chapter 7, Bagman and Crouch).
He may not really know how to put up a tent but he does at least know that Muggles go camping and that putting a tent up with poles, pegs and a mallet is possible. How many other witches and wizards could say that?
"Ron, anti-Muggle security!" said Mr Weasley, his face shining with anticipation. "When real Muggles camp, they cook on fires outdoors, I've seen them at it!"
(Goblet of Fire, Chapter 7, Bagman and Crouch).
Again, he's actually observed Muggles camping in the Muggle world. How many other witches and wizards could say that?
If Arthur Weasley seems dim it's because the rest of the wizarding world is totally and utterly ignorant. Perhaps it would be reasonable to expect him to know more than he does. But he's one of the only people (if not the only person) who takes an interest in the Muggle world. He's mocked and chastised for doing so by his wife, and the rest of his society. The ways of Muggles may have been both his profession and his hobby but he was really an oddball as far as his fellow-wizards were concerned. His passion could never really evolve beyond an eager, nerdish curiosity, no matter how much he might have liked it to.