I'd like some insight into my MIL's strange behaviour

EDIT - thank you all for your helpful explanations. I feel so much better about this situation.

Hello. I'm new here and although my MIL stories are nothing as dramatic as what I've read here, I figured this would be a good place to get some insight into my MIL's puzzling behaviour.

Preface: my hubby moved 15 hours away from his home 15 years ago to look for work. In that time he met me and we set up a nice little life for ourselves. Due to distance, we see his parents once every 1.5-2 years or so, and no more than a week at a time. I've noticed an odd pattern of behaviour that baffles and upsets me.

MIL has done things that seem to be very disrespectful of her son's feelings and time. As a child, her and his dad would not bother to pick him up on time from soccer practice and leave him stranded for hours at a time. Once on vacation she stranded him and his son in the rain, two hours late for their meet-up time because she was shopping. When we went to visit, we arrived to their empty and locked house after a 15 hour drive. We called them when we were an hour away, they left no note, no doors unlocked, didn't bother to call, and waltzed in an hour later with a very forgettable excuse and no apology. There are more stories about her and how she treats others that give me this overall sense that she simply puts her own needs above others'. It's important to also mention that it's like pulling teeth to get them to visit us, but when the Queen was in town they couldn't come fast enough.

But this is what I don't get: She goes into hysterics when we say our goodbyes. I mean sobbing on my mother's shoulders at the airport, begging her to take care of her (37 year old) baby. (My parents live close by and we see them often). I mean sniffling the day before our wedding that a mother is a son's first love. I mean bawling on the phone over Christmas that she misses him so much. She puts on this big damn show about how much she loves her baby. But every time we see them she does something neglectful and careless, and her histrionics piss me off to no end because I see how much her behaviour hurts my husband. I mean, if you miss your child to the point of tears, how hard is it to arrange a visit? And when we do visit, how hard is it to ensure we don't have to piss in your backyard because you just left to go somewhere because you felt like it?

So - any ideas as to why her actions are so incongruent with her feelings? My husband arranged for them to have a visit here and it's stressing me out.