They are just too many domains to register. The domain name steamcommunity has 14 characters. Users could:
Leave out one letter (+14 domains)
Miss a key and type the letter left or right from it instead (+28 domains)
Miss a key half-way and hit the key next to it as well (hsappens to mre all the timew). This can be the left one or the right one, and either the intended or the additional character can be first, so +56 domains
Switch the order of two letters, for which there are 12 possibilities in this case.
Intentional replacement of similar characters as frequently done by phishers, like in the example. Replace o with 0 or rn with m or adding a -. There are 5 places where one could do that in the above domain name, and they would need to cover all possible combinations. So 2 to the power of 5 minus 1 = 31 additional domains. And I haven't even talked about punycode yet.
So we got not one but 142 domains to manage now.
Oh, and of course we don't just need the .com. We also need all the other top level domains. We need all the country-codes, the generic TLDs .com, .net, .org, and I heard they added a couple new TLDs in the past years. How many are there now? Can't be that many, right? https://en.wikipedia.org/wiki/List_of_Internet_top-level_domains ONE THOUSAND FIVE HUNDRED AND TWO?!? Please, IANA, you guys got to learn how to say "No".
And domains are not entirely self-managing. Making sure all those domains work, are paid for, are securely configured and have up-to-date certificates is a job which takes several person-hours each year.
And for some of these domains there might be a legitimate use-case of someone else. Perhaps I want to create a community for scientists and enthusiasts in the STEM field called stemcommunity.com? Too bad, I can't have it, because Valve wants it (actually, https://www.hugedomains.com/domain_profile.cfm?d=stemcommunity&e=com ). That means Valve would need to constantly negotiate for domains which are similar enough to theirs. Another money-sink and time-sink, and time is money.
So bottom line, it's simply not practical.
Especially considering that most users no longer type in domain names completely. They just enter the approximate name of the website into their browser address bar, and let their default search engine add the rest. So the most efficient measure against typosquatting is something any decent tech company is going to do anyway: Good search engine optimization.