> I've hopped jobs about 5 times in my career and saw a 20-30% pay raise each time

> What incentive existed to unionize in that environment?

These situations are not incompatible.

> Unionization only makes sense if you think the good times won't last.

"Good times" is a soft term that translates to "when the employer's short term goals line up with your specific employee goals"...ostensibly to keep getting paid those big numbers.