Economics

• Pools became more centralized for reasons of economic and energy efficiency. Pools themselves do not actually have any hashpower, they simply relay the results of mining to the rest of the network.
• They have never performed a 51% attack because every incentive is aligned against doing so.
• Pools have been getting more decentralized in recent years due to protocol upgrades.
• What you can do with a 51% attack is extremely limited. You can't spend anybody else's money and you can't print BTC which isn't meant to be printed. The only thing you can theoretically do is a double-spend, and pulling that off is basically impossible due to the amount of value you need to transfer extremely quickly, nobody being able to hand you a billion dollars of oil in 30 minutes, and needing to wait for multiple block confirmations for a purchase that large.
• Anybody in any country with electricity can mine Bitcoin. If you want to be real efficient about it, you spend a few hundred to a few thousand USD for an ASIC
• Energy is the most equitably distributed resource in the world. It falls from the sky. It runs in every river and gust of wind. It is in the crust as uranium.

Also, why is having a set unchanging hard limit on total supply that important?

Because otherwise somebody else is printing that currency and transferring value from your currency to whatever they want to do with it. And if we give an authority the ability to print currency, they may be a great custodian of that trust until they aren't. Historically, for the most part, government haven't been, they've just been the best solution so far to the problem of who to trust with the currency supply because they were the most stable institutions humanity had ever built.

Bitcoin's value relative to the dollar is unstable, yes, but it's more stable than many other currencies out there. And it's stable relative to its own supply. 1 BTC will always be 1 BTC. No fiat currency can make that promise. Stability improves with scale, the more people use it, the more stable it becomes, same as any other currency.