It doesn't exist yet because the regulations didn't (until recently) allow for any graduation between a seat that goes up to 13kg (fitting roughly a 1yo) or a seat that goes up to 18kg (fitting roughly a 4yo).
In order to get something that passes crash testing and can safely contain a 4yo who is much bigger than a 1yo, you need something relatively sturdy, even the "lightweight" ones weigh about 5/6+kg.
Then you've got to work out some way to attach it to a pram chassis, which isn't a simple ask. And this way to attach can't interfere with the mechanism for fixing it into the car. Very few pram chassis(es?) are going to cope with 24+ kg of weight attached to whatever small attachment they have for the pram seat which needs to be removable for folding, even if you invented a new kind of car seat adapter - it would still need to fit into the same slot. A car seat with wheels like the Doona might be more practical. I think there's one like this in the US actually, you pull it behind you like a suitcase on wheels.
And then in order for it to be useful, you've got to put a carry handle on it - most parents are seriously struggling with their baby's infant carrier by the time they are about 8-10kg, let alone a giant 18kg four year old with legs everywhere. It just wouldn't be practical (nor can I imagine a carry handle strong enough!) I couldn't carry my 13kg 2yo in anything, I don't think. I can carry him alone but trying to carry him + 6kg of car seat from a single handle would be really unwieldy. Maybe if it had two handles, like a laundry basket :o
Now that the newer regulation R129/i-size is out, you can make car seats with in between weight/height limits, so it might be possible we'll see in the next few years something that goes up to say age 2 or 3 with perhaps a 15kg weight limit (like the majority of pushchairs) or a 13kg seat that isn't a reclined infant carrier. That would be interesting. I think it would likely be quite a niche market, though, so it might not be economically worth it.
BTW, it's quite unlikely you'd breach any weight limit using a current 13kg limit infant car seat on a pushchair adapter, as most puschairs are rated up to either 15kg or 22kg (plus pushchair seat weight), which is definitely going to be heavier than a 13kg infant + infant carrier.