The rate is higher because we can "catch" more cases with better diagnosis.

Imagine machine that is throwing 100 balls per second. Another machine that can catch 10 balls per second. You catch 10 balls.

Now newer machine can catch 20, and newer can catch 50.

Does that mean the number of thrown balls is higher? No. It just means we have machines better at caching them. The same goes for any illness, autism, schizophrenia, cancer, depression...

Some ilnesses we are better at curing, does that mean the the illness is getting weaker?