The problem is that if you list on 2 websites at the same time, then these websites cannot guarantee availability anymore, unless there is some protocol to communicate this.
E.g. what if someone books an apartment on website 1, and another person books the same apartment on website 2 at the same time.
Usually hosters that do this are "professional" and have multiple listings or ways to mitigate it. One is tools that auto-update multiple platforms to keep them in-sync, leaving only small "timing" windows where there could be a double booking. Another is to simply phone the renter and offer another place (if they have multiples right next to eachother which is often the case), or give some sort of discount, etc.
Essentially, it's a "solved" problem at this point and is on the level of being a commodity.
This is a somewhat solved problem by hotel reservation platforms. Responsibility to notify the platform about availability changes are on the property owners. They pay some fines or (some other form of cost) if they have to cancel a reservation due to clashes.
Of course this is a solved problem. Even cheap booking systems manage and synchronise a whole bunch of different booking platforms. Tourism is a multi trillion dollar industry.
E.g. what if someone books an apartment on website 1, and another person books the same apartment on website 2 at the same time.