Hi, raised Christian (though not evangelical) in a heavily evangelical part of the US. I'll take a stab at these.
Why do people act like this?
Christians are taught to. Christians, particularly evangelicals, are explicitly taught to "always be ready to share The Truth" (TM, captial letters, because they do indeed conceptualize it to be the singular objective truth, and see "belief" as merely a matter of accepting or not accepting that truth) in any circumstances, no matter how little they can expect it to be welcomed.
Do they know how unwelcome these kinds of comments are? Are they doing it anyway to prove a point?
Absolutely yes, and... kinda, though not always intentionally. The Christian Martyr Complex is real — you are taught that your message of The Truth (TM) will be unwelcome, because [insert bigoted assertions about people who are not Christians here] and they do not want to hear The Truth (TM).
The part they don't tell you is that this rude, unwelcome, and frankly ineffective kind of proselytizing mainly serves the purpose of reinforcing every terrible assertion your Christian Leaders have made about The Non-Christian World when people react in entirely predictable fashion to being browbeaten with an aggressive Jesus sales pitch. "See?" they'll say, "Jesus warned us the world would reject his followers."
Why do they state their beliefs as facts when they know the people they are talking to don't share them? Do they really think they'll convert us this way?
See above about the Christian definitions of Truth (TM) and belief — they have convinced themselves (or allowed themselves to be convinced) that anyone who does not practice Christianity is consciously rejecting the simple facts of the universe, or has been fooled into doing so. "Jesus is the son of God" is to them as factual as "water is wet." Whether they'll succeed in converting anyone is irrelevant — after all, failures are recast as evidence that Christianity has The Truth (TM) the world refuses to hear.
Does it just really offend them that some people aren't into Jesus and they can't compute?
Yes. And more than offend — the really emphatic ones? The ones who really, truly, in their heart of hearts believe they're doing the work of God? They're heartbroken and confused that people aren't eagerly jumping on their invitation to convert. They'll double down harder next time.
Do they know they're being disrespectful and don't care, or do they not know?
It varies, in my experience. Some Christians know it's disrespectful, but believe that an opportunity for people to hear The Truth (TM) is more important than any etiquette. Other Christians simply don't — have been taught not to — see how disrespectful they're being. Both of these tend to take a view of like... "But people need this message. If they understood, wouldn't they be grateful we never gave up?"
This type of Christian doesn't fully respect other beliefs, at the end of the day. They can't. They cannot view Judaism, in particular, as anything other than a prequel to Christianity. Holding this worldview in earnest depends on thinking of every other belief as an active rejection of The Truth (TM) that Christians are called to proclaim, and it fucks 'em up big time.
I have a dear friend who once told me that, as a child, they used to cry and pray that my family would accept Jesus and come to their church, because they didn't want to go to heaven one day and me not be there too.
Someone's going to #Not All Christians this, I'm sure.
My parents raised my siblings and I with a more pluralistic view, with the idea that we believe X, Y, and Z, and other people believe other things, and that doesn't make us right and them wrong, because the world is vast and complicated, and God even moreso.
Even so, I think my parents believe the faith they practice is a little more right. That Christian conception of The Truth (TM) as something singular and knowable — as something special to Christianity — seeps into everything.
I don't have a pithy conclusion for this, and I don't think any of the above makes that kind of behavior any less rude or inappropriate. But that's why. It's baked in.