Because when a hosting company scales up its network to withstand an attack, many other companies do the same. And the attackers open accounts at several companies to launch the attacks.

If you can distribute all your services including DNS, across several providers, then you can probably withstand the attack yourself.

Whatever you do, the attackers will learn and evolve. So the best you can do is to get the pain down to a level that you can withstand. Only when everybody does this, will the attacks stop, or more likely, move to another venue.