They just spent <insert large sum of money> to buy the company you work for. EVERY acquisition, at some level, is a talent acquisition. They're buying not just the IP, but the people who made that happen. They need to have everyone who knows how to contribute on board. There is no point in your career that you will have more leverage than on the day a company you work for is acquired.

Which is why they put out these silly "take it or leave it" offers. They're trying to pressure you into giving up that leverage. When you call their bluff, you'll find you have a really nice negotiating position.

There are exceptions of course. Failed company acquisitions for instance generally don't leave you a ton of negotiating room (although even then it's not zero).

So in short they're going to call in a panic because they need you much more than you need them.

It's not easy to replace talented people.