You are making a legal contract which your employer is trying to extract a maximum amount of profit from, while passing on risk, and has more experience and understanding of how to negotiate to that effect.
Maybe that's true for most venture-funding backed startups, but it's a bit of an unfair generalisation. There are plenty of employers out there that don't want to suck their employees dry. Some companies have a more long term vision, and the easiest way to retain employees for more than a few years is to not exploit them.
Both parties should want a place that's enjoyable to work over the long term, yet sometimes the company will have to make hard decisions. Priorities slip or people are straight-up unable to avoid, say, laying off half the staff. Framing and context matter, as always. "Your employer is not on your side" is hopefully not a statement about the day-to-day interactions with your boss, or even a statement about company values, but it can serve as a reminder that there's always a line somewhere, and, intent aside, your best interests may simply fall on the wrong side.
The very best employers don't just treat their people well, they also educate them.
I think I generalised it correctly. I agree with this point - that it's often not the most profitable to suck your employees dry, as you say.
Your employer is not on your side, even if your goals temporarily align with their long term vision. That is almost the best you can hope for in employment.
If business has a time of crisis, for whatever reason, then it becomes a risk-minimising strategy to exploit staff - at least until the crisis is over or the business collapses.
There are others (with significant overlap) based around retaining market mechanisms, but externalising labour conditions and salary, and so using market mechanisms as a resource allocation mechanism. A market is only a threat to workers if there is a significant negative impact from being made redundant - some socialists believe society should strive for a system where we encourage businesses to make people redundant by making themselves more efficient; redundancy protections are a band aid for a lacking welfare system.
It's not for nothing that Marx and Proudhon violently agreed that one of the most radical policies you can pass is providing cheap credit (though they disagreed about agreeing...) - make credit cheap and you make exploitation of labour far harder because it becomes easier and lower risk to leave to work for yourself.
Sure, state socialist ideas ideologies would have the issue you describe, which is one of many reasons why a whole lot of socialists find them more objectionable than capitalism.