This is because they are doing what is being termed as SPV mining (not normal or should I say "proper" mining).

When you mine in this manner you risk ending up on a fork (as happened last year to the Chinese mining pools after a soft-fork).

So if you had huge blocks and incredibly good hardware (and bandwidth within your pool of miners) then you'd just play games with those that do SPV mining and make them all end up on forks (after a while they would simply give up trying).

I hope you are starting to understand that SPV mining is *not good enough* (and the more you scale things up the worse it gets).