I believe in the UK it's by the card issuers as a security measure (it's definitely a flat £30 for most cards). They're happy to be liable for that amount a few times until the fraud prevention measures kick in, but not more. Apple's appetite for risk is perhaps a little bigger.
I don't think I've signed anything for 10+ years tbh, but the UK went pretty heavily for chip and pin early one. But honestly it's a coinflip whether I remember my pin these days.
The contactless payment subject to the £30 limit do not use the same protocol as Apple Pay. The limit is lifted because it is a more secure protocol similar to Chip&PIN with a random per transaction CVV. By comparison payment with contactless on card is dumber. It’s essentially equivalent to paying online with your card screaming its number to whoever wants to scan it.
If you use ApplePay on an older terminal, which does implement the more secure protocol, it falls back to the same protocol as used by contactless cards and is subject to the £30 limit.
Apple probably requires fully authenticated bi-device on-line transactions making the risk much smaller.
The EMV part requires the merchant device to be online or the process won't happen and the phone part requires user authentication; you'd have to break both at the same time to abuse it. Probably still possible, but much harder than stealing a card and using it until you hit the limit or get flagged.
Probably also why EMV contactless without PIN and EMV contactless with PIN have different limits (amount of transactions and transaction amounts).
I don't think I've signed anything for 10+ years tbh, but the UK went pretty heavily for chip and pin early one. But honestly it's a coinflip whether I remember my pin these days.