Why do people stay in a sexless relationship instead of adopting a pet?

Other than for financial reasons, of course.

I have heard a lot of people say they stay in a sexless marriage because they don’t want to be alone. These people make good money and finances is not an issue. Why not get a pet for companionship?
People, please read the question, I said “sexless”.
Tino, please read before answering

How long do parakeets usually stay in a pet store?

I want to get parakeets(budgies) but I want to know how long they usually stay in a petco.
Also I want to know how old they are when you buy them.