Because Google[1] and Facebook came along and scared everyone by iterating at a LOLWTF pace and companies in surrounding spaces looked at what was taking up time in their release schedules and the answer was "test passes" so they fired all their testers, and told devs to add unit tests but unit tests don't cut it.

Companies that used to have immaculate software quality had dedicated test automation engineers who had the job of abusing software in crazy bizarre ways. Then they hired armies of manual testers to go over anything that hadn't been automated.

Lots of problems existing with this system, one of which was career advancement for software engineers in test was limited because it is hard to get recognized for the two primary jobs of an SDET:

Signing off on code

Blocking a release on quality grounds

So you had a gradual rot of SDET and test orgs at companies, with pools of brilliance that slowly got drained as the best engineers got tired of being undervalued.

Start from that base, and then around ~2010 everything needs to start "moving fast".

Apple and MS both get rid of their test teams, and with two of the largest employers of dedicated software engineers in test getting out of the field, the entire field itself falls apart. Now it is career suicide, an ever shrinking career path that pays far less than doing "real" development work.

That leaves us at where we are today. Everything sucks and breaks all the time.

[1] Everyone forgets how bad the first 5 major versions of Android were.