There is a whole Bunch of reasons why _not_ using a Laptop for this.
A Laptop means one more device to unpack, setup, power up, remove and pack on each Gig, consuming valuable time. It means one more bag in Car, one more source of mistakes and breakdowns, two more Cables (Power and USB) which can be forgotten, broken or cause accidents.
Especially with Rack Mixers this _are_ valid and important Arguments. Those Mixers are chosen because the user wants to keep his gear compact, highly mobile, easy to use and fast to install/remove on any stage. Any space usage for a Laptop (and there is no Laptop that uses no space) is too much here.
Needing a Tablet for UC Surface to control the mixer is another story. That tablet will be connected via WLAN in most Cases and either operated from one of the Musicians on Stage or from a Technician sitting in the audience. There is obviously no way for these to operate an additional Laptop connected via USB.
So it is definitely a caveat for that mixers to not support multitrack recording.
Additionally, it is absolutely not plausible that enabling multitracking would make those mixers significantly more expensive. The Hardware is already there, the core of that Rack Mixers seems to be absolutely the same as the core of the Desktop Mixer Series, just lacking some physical Controllers. And those Desktop Mixers _can_ do Multitracking - so the Software for this is already there, too. There is no plausible need to extend the Rack Mixers Hardware (except maybe a faster Card reader but that won't eat much money since such readers are available widely as cheap mass poroducts) and no plausible reason why the already existing Recording Software from the Desktop Models cannot be executed on the Rack Versions. So any higher pricing for multitrack recording is a pure political decision, absolutely nothing else.