As an old wise caravanner, let me give you the best advice you'll ever get.
BUY A SMALL TROLLEY JACK.
It will fit in the boot of any car, or under a locker in your caravan. It will be worth it's weight in gold if you get a puncture on either your car or caravan. It will be extremely useful if your jockey wheel gets completely bogged down, and you need to get something firm under it.
I entirely agree with you that manufacturers should supply a jack.
The fact is though, that most 'supplied' jacks, are cheap, nasty little affairs that are not very safe to use, and are a complete fiddle to mess around with.
Imagine the scene.
You've just had a puncture on your caravan.
The offside wheel. Motorway traffic thundering past you at crazy speeds despite the rain, wind and impending darkness.
And you want to mess about with some "Made in China" jack that could collapse, or suddenly slide away when the gust from a big lorry rocks your caravan?
Not me!! No way. Not ever.
We take our small trolley jack on EVERY trip, and it's been worth it's weight in gold. Cost about