Because not sharing the benefits of increased productivity with your employees discourages them from adopting new technologies.

If I'm 25 then sure, probably I need to learn the new technology anyway because my old technology will be obsolete before the end of my career.

Still, if I don't like your new technology, I can just switch industries and you lose an employee. You want to force all employees to learn Python while I'm okay with keeping everything in Java? Oh well, I'll find a Java job then, good luck training someone to replace me.

If I'm close to retirement, then unless you pay up significantly, there's zero reason for me to learn the new technology, and I'll resist as long as I can.

Before we got an infinite amount of AI-generated Indians, software wages were sky high exactly for that reason - in order to incentivize people to learn new technologies (coding).

Of course when it's the employer's market none of that applies and you just suck it up. But again, because minimum wage exists, if I'm working for minimum wage anyway, I'll simply choose the least demanding job where I don't have to learn shit.