Viruses used to be largely statements and annoyances, the sort of thing a rebellious teenager would pursue as a form of computer vandalism.

Then people realized there was money in botnets and password stealing. The goal of those viruses is to get onto your computer and stay there as long as possible. If you notice it, you'll remove it, so it is in their best interest to avoid being noticed.

Unfortunately, a lot of them are so poorly written they are hard not to notice.