Because NATO has the initiative here. The guarantees, unofficial or not, now function as a deterrent for possible Russian aggression. A promise/threat to join a hypothetical future conflict is very different from joining an existing conflict. One is a deterrent, the other is escalation.

Besides, Finland has been a close NATO ally for a long time, shares NATO values, our military is already 100% NATO compatible, and we are strategically and geopolitically a pretty important piece of the puzzle.