A small startup has a finite amount of time to either become a big company in their own right or do something so noteworthy that a big company sees the need to acquire them. Nothing else matters. There are minimal incentives to invest in the long-term welfare of your employees because in the long term, the company doesn't exist. You can't even guarantee that an acquisition will keep the employees you have invested in.

Large corporations like Google are incentivized to give their employees reasons to stick around. They can expect the company will be there in 30 years, and they can expect a good employee to put in a career's worth of work for them (and eventually have peer and mentorship contacts that encourage other good potential employees to join the company).

This is painting with a broad brush of how the incentives are structured... Not all big companies see it this way and not all small companies see it this way. But it's the behavior the marketplace appears to reward.