Why can't an encapsulated and dehumidified crawl space have vents sealed off?
The reality of code is that it is written by a group of people who are influenced by tradesmen, lobbyist groups, government organizations, etc. Is the international residential code always going to allow for all acceptable solutions to all scenarios in all locations where it applies? Not by a long shot. They pick and choose methods, sometimes get things right, sometimes wrong (and hopefully fixed in a future revision as has historically been the case with a lot of requirements for basement insulation techniques).
If some new doohicky comes along where a company stands to make a lot of money from a code change to allow the use of the doohickey, a company can lobby the council in charge of the code (or if it’s a large company, they may have a sitting member on the council) and possibly get their special interest addressed.
Without official published reasoning behind the code being the way it is, the best we can do is speculate.
So, right or wrong, here is my speculation:
I think this scenario is uncommon enough to date where the council has currently settled on an “all unfinished” or “all finished” approach to the treatment of crawl spaces. You could make a similar argument about requirements for attic spaces.
I think one of the primary reasons could be that a standalone dehumidifier like you are suggesting is a small appliance not usually installed by a builder as part of new builds where air conditioning equipment is (toaster vs oven kind of thing). Since the dehumidifier is not built in, you could move out and take your dehumidifier with you and then the next homeowner would be none the wiser that a dehumidifier is required. Therefore, I think the requirements are in place to ensure the space will be addressed passively by vents or addressed actively by something expected to be a permanent fixture of the home that you or a future homeowner will notice if it stops working (a homeowner would likely notice if the furnace or air conditioning fails).
It is what it is for now, but if you or the wifi dehumidifier lobbyist group wants to go to bat, you could see a change in the next revision of the code itself.
Another option, depending on your plans and locality, you could always request special dispensation from the Authority Having Jurisdiction (e.g. local building department) and they might grant you an exception.
Also, you didn’t quote code verbatim and I do think there might some wiggle room for using a zoned air conditioning approach where the crawl space could technically be a different air conditioning zone. Again, this is open to interpretation and the Authority Having Jurisdiction has final say.