I think the answer is simply that the house name gives the impression that the quality of all the stories in any particular issue will be the same as the quality of the stories in any other issue. Pulps were sold as a commodity ("Contains an entire short novel!" "160 pages of thrills," etc.), so anything that suggested variability (like different authors this month) was perceived as a negative. It's also likely that publishing houses wished to make authors disposable - and the way to do that is to make sure that if "Joe Smith" (writing under the house name of "Richard Hardjaw") demanded a raise or became otherwise a problem, "John Doe" could slip into the job of being "Richard Hardjaw" without the customers having a moment of doubt.
This article on the subject confirms the second reason:
In the pulps, I’m not sure when they started to be used, but the first
I knew of its use is with “Maxwell Grant” as the author of The Shadow,
rather than credit Walter B. Gibson. Interestingly, Gibson created
that name, using the names of two magic dealers. When you keep in mind
that The Shadow, like the previous dime novel characters, was owned by
the publisher, this is why. This was different from the many previous
serialized characters that were owned by the authors, as well as you
now had a magazine focused on and named for the character.
Hence the main reason for the use of house names was that should there
need to be a new author to write the character, this can be hidden
under the house name, and so most, but certainly not all, of the
character pulps to follow did this. Making it more tricky is when
these house names would be used for other characters, or would be used
by the publisher for more wider use.
I should also note that house names were and are common in markets that appeal to the same folks expected to read the pulps - like "Franklin Dixon" for The Hardy Boys, "Carolyn Keane" for Nancy Drew and "Ellery Queen" for the Ellery Queen books.
