I know a guy that works at a very well-known tech company and has been a software engineer for over 15 years for them and is only being paid $120K.
He says "They treat me well", and I'm like, no they fucking don't. They don't value your work anywhere close to what it should be. I told him that he could easily triple his TC by going almost literally anywhere else.
I used to work for said company. I left because the salary was way below market and literally doubled my salary from $100K to $200K at my new digs. I made it no secret to my old manager that money was the #1 reason why I was leaving (#2 being that the team I was working on was basically falling apart due to internal politics), and he acknowledged that salaries are significantly below market and that HR is promising updates to salary bands to bring them up to market within the next couple months. Spoiler alert: It's been 1 1/2 years since I left, and the couple people I talk to from there say nothing has changed.
Just remember that these employers underpaying their staff are going to be the first ones on HN saying how unions are not needed because pay is already high in the tech world.