Because I am uninterested to begin with, there is no need to assume I am and give me the hint.
Well, maybe it's not necessarily about you at all, it's about them being comfortable with the boundaries they set up around their own lives.
They can't read your mind, so they don't know whether to assume you're interested or not interested. In the past, I used to assume that men weren't interested in me by default but had a number of awkward experiences, so now am more reserved on the basis that what I perceive as normal friendliness on my part can be misinterpreted as interest by them. This is because I prefer that someone doesn't get the wrong idea early on and thus avoids an awkward situation down the line.
There's probably a range of reasons the various men you've spoken to did this –some might be trying to put you at ease, some might be trying to put you off, some may have genuinely meant nothing at all by it.
The more important question is "Why do you care?" Why are you annoyed by the thought that someone might have mentioned their wife or girlfriend as a way to establish their marital status to you (and quite possibly other women)?
If you're not interested, the information is relevant to the same degree that he might have mentioned he has a dog or likes the colour green.
I talk to a lot of men in the course of my work who I'm not interested in pursuing and I honestly can't remember when they first brought up their significant others in most cases. The existence of their wives isn't relevant to me, so learning they have one becomes one of a collection of emotion-neutral facts I know about them.
It's interesting that the mention of a wife or partner by a man does trigger an emotion with you – why do you think that is?