> why can't Google just ignore spammy links instead of penalizing the whole websites and all the websites that link to them

You are suggesting an approach where there are no downsides to link spam. Obviously, players of this game would then ramp up their link spam bots, and run them non-stop. Because it causes no damage to the site they want to rank.

Link spam isn't just a search engine indexing problem, it is also a detriment to the general Web user too. It makes the Web worse, not just search.