Because the acquirer wants the company's talent i.e. founders and employees.
Because the acquirer wants to deny a competitor the opportunity to buy the company.
Because the acquirer is trying to put the kibosh on a disruptive innovator. In other worse, keep the genie in the bottle—or shove it back in.
Because the acquirer wants to "inject new blood" into the organization. (Think about the NeXTers charging the gang planks and taking over Apple after being acquired…)
Probably others.
a) Google can't find people as smart as those at Wufuu and PushLife.
b) Wufuu and PushLife might get picked up Yahoo (search) or Apple/RIM (mobile).
c) Google feels disrupted by Wufuu and PushLife.
d) Probable, but the cost is not justifiable for "inject[ing] new blood".
In my company we often buy-in technology because, even though we could write it ourselves, and might do a better job in the end, we are pretty sure that there will be hiccoughs and unexpected difficulties, and during that time we will be dedicating manpower that could be used doing things that other people can't replicate.
dl;dr: Risk and time.
I am starting to think they are buying the founder's passion and nothing more. Which evaporates soon as s/he is bought out but that is why they make sure you stay on a while and show them the ropes, after that time a replacement is chosen by the founder I suppose.
Most of their acquisitions are those little guys.