The OP has a point - why do these creeps attach themselves to her personally?
Because she let's them.
These people don't reserve behaviour for the OP and people like her. Many of them won't even recognise that what they are doing is considered wrong by others - it just meets and serves their own personal needs.
So that's how they are with everyone. And even then, some of it isn't 'bad' (eg see the example I gave of my partner). Thoughtless at most.
The difference is that everyone has different boundaries in terms of what they are willing to accept from other people. When someone else behaves in a way that you don't like, you find a way of communicating that to the other person. That is your boundary.
If you don't, then you're communicating that you don't mind it and so they do it again. The longer it goes on for, the more it feels like they are 'pushing' your boundaries. But if your boundaries are only felt by you (eg discomfort and feeling like someome is taking the piss) then resentment builds up in you. But the other person is still unaware.
And so they push. Not because they're arseholes but because it meets their needs and they assume that, if you're not happy, you'd let them know.
Some people are complete pisstakers so they'll be themselves and a lot of people just won't be there or put themselves out for them. Those people perceive them as unkind/unhalpful/rude and so dont bother with them again. Someone else does accommodate them, out of a sense of kindness or wanting to be nice or their biggest fear is being thought of as rude and not liked. They just feel silently disregarded, disrespected, put upon.
This encompasses all sorts of behaviours from people who have friends who are persistently late, to people who ask for last minute babysitting favours, to people who just take advantage of your good nature, to men who cheat.
When you accept someone's behaviour, you are showing them how you to treat you. As I said, people's behaviour will push to the limits of what you will accept.
The more you accept, the more they will push and the less they will appear to respect you. Not because they have deemed you unworthy of respect but because they don't even consider whether they are disrespecting you because you've never put boundaries in place that they can disrespect.
People can't respect you if you don't respect yourself.
If you're quietly seething that X always does something or never does something else, you have to consider - have you actually communicated it to them?
It's why people start to lose friends when they finally start having boundaries. They've moved the goalposts - changed the parameters- and it now no longer suits the other person.
It's easier to establish boundaries from he start. The only people you'll lose or exclude are the ones not prepared to meet you needs.