The Death Star I wasn't easy to destroy. The thermal exhaust port is only 2 meters wide, and it would take a starfighter near-ideal conditions to successfully fire a proton torpedo in. Add to that the base defences in the form of turbolasers and TIE squadrons, and only one other pilot besides Luke managed to find an opportunity to make a trench run before Darth Vader arrived - his torpedo didn't make it because he was under immense pressure in a race against time. In pretty cliché fashion, you're underestimating the power of the Force (in overcoming battlefield stress).
If Darth Vader wasn't there, if the Rebellion had enough fighters to throw at the Empire, and if there was no time limit, the Death Star may eventually fall without the Force, but the Battle of Yavin was none of that.
The Death Star II was actually indestructible once complete - it's many times bigger, there is no more vulnerability like the thermal exhaust port to exploit, it had proportionately more surface defences (turbolasers) and TIE squadron complements than the first to combat starfighter threats and the superlaser can be calibrated for energy-efficient destruction of enemy capital ships for proportionately lower cooldown instead of the Death Star I's cooldown of 24 hours. It's indestructible when complete, but Palpatine wanted to use its apparent weakness when under construction as bait. That backfired spectacularly, but you can't use that as argument the Empire couldn't build an invincible weapon - it would have been.
As for Starkiller Base, I can't be certain, but it's possible the First Order got overconfident - if something is the size of a planet instead of a moon, surely it'll take so much more to destroy it, right? Still, it took exceptional skill to destroy it.
All the starfighters the Resistance could throw at the First Order wasn't enough to destroy it on their own - they're only scratching the surface and the TIEs that scrambled to defend the base basically picked off half of the Resistance fighters before the situation changed. It was thanks to Han & Chewie's explosives that opened a hole which permitted only one with the skill of Poe (the best pilot of the Resistance and maybe the galaxy) to fly through and destroy it from within.
One should not judge a weapon's ability to stand up to external attack based on its ease of falling to an internal one, I say.