Because there are institutions that rely on them. Things like air traffic control, the stock market, etc. run on mainframe environments. If it ain’t broke, don’t fix it.

Real people doing real work. Next up, why is my hospital’s EHR running on POWER?

(2015)

But yeah, because people are still buying them and they're super-profitable.

(2015)

And the answer is, because they make money.

Why do they make money? See garmaine's answer.

[Edit: Rats, wmf beat me to both my points...]