The gist is because DNS gets updated. Your computer already has a local cache of all DNS responses it receives and relies on those until that entry expires (as set by the TTL field). If you want to run a local resolver, you can, but it wouldn't solve this problem, because you'll still need to update your cache regularly and guess how you do that? With DNS.

The next question from there might be why do DNS responses change so frequently? I don't have an exact statistic, but most of them don't (I would guess 90%), so you could probably indefinitely cache those. But the ones that do, iirc, it's usually to allow quicker changes (esp. if the site is under active development), load balancing, global distribution, etc.

I hope that's helpful.