How hard to do ATSC and DVB?
Many companies pump out DVB, ATSC, CMMB, ISDB etc demodulators for the (very price concious) TV and STB market, it makes no sense for this kind of product to use anything else.
The few multi-standard demodulators that do exist are generally software defined receivers, these are very expensive in comparison and generally only go into expensive test and measurement equipment. Look up Rhode and Schwarz test equipment as an example.
So, it's not very hard to do but it is very expensive.
Then you have to think about why you'd want to make such a thing, the number of people who need multi-standard receivers is pretty low so you end up with an expensive thing that only is attractive to a handful of people, not a great business model (unless you're R&S).