They let you do it because GitLab was first to let you do that. MS had to match the offering of their most noticeable competitor. They also felt like thay had

They let you do it because GitLab was first to let you do that. MS had to match the offering of their most noticeable competitor. They also felt like thay had to one-up them... well, to win the competition. So, from the sales point of view, Github Actions was, at the minimum, an alright idea. Not brilliant, but quite obvious and expected. From the engineering standpoint, however, this is a disaster on many levels. But, that never stopped Microsoft before. They don't try to win the market by making an objectively better product, their tactics are and always have been to make a product that can claim (with an asterisk) to be able to do a lot of things the customer wanted only to discover afterwards that those promises were phony.