You can't force them to care. Maybe they weren't a close family and have different ideas about what family members should behave like and so this is why your husband doesn't care. For example if his grandparents were like this, maybe he doesn't see the problem?
I think it is weird though. You need to talk to your husband and say basically that it isn't common and you're not happy with it - you feel like your kids are missing out on a relationship and it makes you sad. And that either he can talk about it and ask them to make some effort with the kids or you will stop making the effort with them in rethrn, i.e. no present buying at all, he can take over everything, and visit them alone if they ever do want to see you!
Honestly though, it sounds like they just have different family values and expectations than you do. I have this issue, (although slightly less extreme!) with my in-laws. I love them, but the level of contact and communication that is normal to them is too much for me as my family are ready different to them and I'm just not used to it. I'll expect they won't even really know there's an issue - for example, if they send birthday presents, but don't see the kids on their birthday, they'll probably be shocked that you're annoyed as they sent birthday presents, and maybe in their view, that's enough.
I'm guessing you're never going to have the relationship you want with them and honestly, that's okay. No kid ever suffered much from one set of slightly disinterested grandparents. If it was a parent yes, it would be a huge issue! Or even both sets of grandparents would be a bit sad for them (but nothing life-ruining!). But this really isn't the worst thing, and I'm sure they'll be just fine either way. I'd try to stop letting it bother you so much.