A due date is like a due month really though isn't it? That makes it so much harder in this situation for parents in law to plan and it would be less worth it for them to go out to see their other family members over the summer. It's also a very long way to go and very expensive and the longer the trip, the more you can justify the cost.
This is a very different situation to if they just couldn't be arsed or they're choosing to visit their daughters in the town next to yours.
There are also, as lots of posts refer to, plenty of people who have no grandparent help at all. With the fourth, the help my parents needed was someone to look after me and my other siblings. Do you have friends who can help with this? I am someone who would be very happy to help another friend in this way.
When my best mate had her baby, her mum wasn't in good health, her dad's not around, and she didn't have a partner, so she had to have help from us her friends. Between us we did housework, fed her cat, gave her lifts, changed nappies, looked after the baby in her house while she napped in recovery, birth partnered in hospital etc.
Oh hang on - I've just re-read your original message and think I've focused too much on the birth part of the support you wanted from your in laws? Do you mean holiday childcare more generally and for the years to come?
I feel like that's maybe something that should/could have been discussed before having another child? But I might have the wrong end of the stick.
Also, I think it's absolutely fine to tell your parents in law that you're feeling hurt that they won't be around to support you and meet their new grandchild.
I've got a cousin who's 1 of 3 and the other 2 siblings both live in gorgeous places abroad so her parents have a wonderful time visiting them and she can feel like she ends up with the bum end of the deal, she gets the boring end of things, she's the one who has to help with getting them to the garage when their car's in for an MOT or catsits while they're off on their jaunts. So let it out, let them know how you feel in a grown up way.
Oh also, I've spotted you've sensed the lack of empathy in a lot of mumsnet replies and a fair bit of viciousness. That's sadly the norm. Someone can ask a completely reasonable question like "am I unreasonable for thinking egg sandwiches are the worst food for someone to eat on a train?" and by the end of it they're being called anti-vegetarian and racist and a bad parent and ableist. It's wild but it happens all the time. Finally, I'd recommend having a chat with friends about it as well as they'll get the context and should be able to give you some good advice (as well as the practical support they've already offered!).