Because it's beneficial for the software company's reputation. People are more likely to buy the software when they know that it's not going to get a permanently unpatched zero-day the moment the next version comes out.

That model always had the tacit agreement that the company releases early, and the users accept that they are part of a large testing base with one or two major updates to come. Further to this, continued support in the early life drives more sales. There's a spectrum of users from bleeding edge to 4 versions behind. Some will hold out and never upgrade if key bugs remain, so updates make business sense. Software of this complexity has to be this way to strike a balance to move new features forward.

In this day and age people expect security and operational patches. It’s hard work maintaining software, even if it is feature complete.