Because Berkeley, Stanford, MIT and Harvard produce radically superior computer science students vs the average community college.

That doesn't mean no talent comes out of community colleges. It means the average ability level is far lower and the elite students are far more rare.

The school doesn't matter much if you have the ability and can demonstrate it. The kind of grads coming out of elite schools that Google & Co are recruiting, tend to have both the ability and the school. The schools, and the professors there, help more often than not with filtering.