Because the purpose of Bitcoin is to build a currency that substantially eliminates trust.  If Bitcoin users are forced by technology have to trust miners to do the correct thing— to not inflate the currency, to not destroy its decentralization, etc— then they can be easily disenfranchised. Not only would they be forced to trust (which is against the goals of the system) but without the enforcement by a great many users miners would have reduced incentives to not cheat in the various ways that the enforcement prohibits absolutely. E.g. why not increase the subsidy to 26 BTC? Because it would undermine confidence? pfft. They'd justify it by some argument about lost coins, expanding economy, importance of security and such, just like the inflation producing governements and central banks do. Doing it would benefit all the miners, so why shouldn't they "vote" a raise for themselves? And a little bit of inflation at a time demonstrability doesn't undermine all confidence in a currency.

Of course, you do have the option to trust miners in exchange for a reduced validation cost to you by using a SPV node— but that option is only really viable because the miners are regulated by the many other participants who do verify.

Why have mining at all? Because we can't accomplish a decentralized currency without a way of providing for the ordering of transactions, and we don't know a way to provide ordering without some kind of vote or without centralization. So mining is just an attack resistant way of voting on the order of transactions. But we don't use mining for more than that because voting is not actually a good solution, it requires a kind of trust (though better than centralization)— and so for the non-ordering things that can be validated independently nodes do validate them independently. (Even SPV nodes should and do validate all that they can within the context of their permitted operating cost).