April-August of last year, HashiCorp was regularly above a 20% premium over Monday's close. Many investors might think it would get back there without a merger - and it had been higher. IBM is offering $35/share which is close to the $36.39 52-week high. In some cases, investors are delusional and just bought in at the peak. In other cases, a company's shares have been under-valued and the company shouldn't sell itself cheaply.
I don't think one can really have a fixed percent premium for acquisitions because it really depends. Is their stock trading at a bargain price right now? Maybe people who believe in the stock own a lot of the company and don't have more capital to buy shares at the price they consider to be a bargain - but would vote against selling at that bargain price even if they can't buy more. They're confident other investors will come around. An acquiring company wants to make an offer they think will be accepted by the majority of investors, but also doesn't want to pay more than it has to. If the stock has been down and investors think it's a sinking ship, they don't have to offer much of a premium. If the stock is up a ton and investors sense a bubble, maybe they don't have to offer much of a premium. If the stock has been battered, but a lot of shareholders believe in it, then they might need to offer more of a premium.
