Why don’t they have much cross compatibility?

The thing is, most packs have some cross pack compatibility when the come out. The majority of packs take into account the existence of pets and seasons. Some, though not all (My Wedding Stories is an example of that) do refer to activities from other packs. But in general those are only really small things, like being able to buy certain dishes from a previous pack at the foodstand in the newest pack. CAS traits and social compatibility seems to integrate quite well. Lovestruck does have three optional dating activities that require other packs (a lot of people are salty about those showing up for everyone - which is a different discussion). 

But, it's rarely the big things. For example MWS, and Lovestruck too, seem to have forgotten the existence of Spa Day and that the spa is a great place for, say, a bach party, or a romantic date. I'm sure there are countless other examples like this. 

What bothers me most though, is the fact that existing features from older packs never get updated for new packs. Even the new food dishes frequently need to be reported as a bug/oversight before they're added as menu items for Dine Out. The holiday traditions for Seasons haven't been updated for snow sports from Snowy Escape (seems like a nice thing to dedicate a holiday to) or horse riding, if you have Horse Ranch. 

So, there's definitely a lot of room for improvement. 

I don't own Lovestruck or For Rent, but I've heard similar feedback about them. 

But regarding Home Chef Hustle, I disagree. For one thing, it's only a stuff pack, so it's supposed to be niche, and expectations for cross-pack play should be lower than for a full expansion pack like Lovestruck or FR. Nevertheless, I think it has a ton of cross-pack potential. I view it as Cottage Living 2.0, actually. It gives me more of a reason to grow, raise and buy various ingredients, and it helps me use up a lot of the produce my sims grow in their gardens. Sims (even kids) can sell foods from other packs at the food stand (popcorn, lemonade, fizzy juice, etc.). Even if you only have base game, it adds a lot of recipes and gives you a reason to grow and use fresh ingredients. I was getting bored with cooking as an activity before HCH, but now it's one of my favourite things to do in the game. I absolutely love the pizza oven and waffle iron. It's the last pack I bought at launch, one of only two or three I've ever bought at full price, and one of the few packs I would buy again without question if something crazy happened and I lost all my DLC. 

No clue why there isn't more substantial cross pack compatibility features, but I do enjoy what's included so far. They make the game more fun. It'd be a golden opportunity if pack refreshes ever return...