How many times has DC rebooted it’s universe?
The ones I can think of:
Crisis on infinite earths
Zero hour
Final Crisis?
New 52
Rebirth
Showcase #4 (Also, Rebirth isn’t technically a reboot, but eh, semantics really).
Why isn’t it a reboot?
A reboot, at least how I see it, is a universal restart of continuity. Rebirth was just a retcon of some elements of the previous reboot (almost entirely undid the reboot for the Superman books, but for, say, Flash, they just re-introduced some characters previously rebooted out of existence).
By that standard, Zero Hour isn’t a reboot, either.
They all suck, though, reboot or not.