They get work for the same reasons Scott Lobdell and others keep or kept getting work..
1) Easy to work with
2) Get content turned in on time
3) Can do more than one book
4) More willing to put up with the antics at both companies
5) Some folks are bigger names elsewhere that even if the floppy does not sell-their name is enough to get the trade on the store, library or school shelf. See I Am Batman.
And this is where it gets murky...
DC and Marvel are trying to get new blood but it's hard to do that when many writers of genders and demographics are DISMISSED before the first preview is out.
With too many folks looking for REASONS to not bother with the book or stock the book.
How many badly done DC and Marvel books had long runs with bad writers that still moved products? How many books did Lobdell do in New 52?
We want new blood we have to give them shots.
We want books that are NOT using another name-we have to give them shots. If this is a Plantary book needs to have that title. Or we keep seeing old names being used over and over again.