It’s business, big business and sadly enough over-saturated with willing labour! Welcome to the video games industry …
Valve made Portal? Well, technically they did, because they had their name on the title after having bought NMS (Nuclear Monkey Software), a team of young students attending DigiPen Institute of Technology. These guys made Narbacular Drop and Valve bought the full team to make a profit for them. Basically Valve saw a cheap opportunity for a promising title under their name. Cheap labour for Valve to hit gold with Portal, great opportunity for willing students to get their foot into the industry…
Valve ‘Corporation’ just sticks to what is successful, what works best for them (financially), with the least risk involved. It is no wonder they became the biggest software distribution platform. They are good at hiring people to make a profit under their name. Several other popular titles are an example of this. Team Fortress, started out as a Quake modification, then QuakeWorld by TF Software Pty. Ltd. Valve saw another opportunity and just acquired the team … leading to another success! Counter-Strike started as a HL mod, before Valve bought the intellectual rights. Left 4 Dead’s development was started by Turtle Rock Studios, to then be bought by Valve.
Half-Life was their first success, no doubt they had a great team making this one of the biggest titles ever. This success changed their ways however, finding opportunities with alternative ways to make a profit, software distribution. No doubt they intended to continue the franchise which they partly did. I do however think that it didn’t always quite work out the way they planned. One or more attempts must have eventually been turned down or have become too expensive to never see the light. I also believe that over the years, likely too many hands are in the pot of what many (still) believe to be ‘gold’. Valve just decided to go silent and stick to the legacy as a marketing tool. Best to not risk anything unless an opportunity truly arises, which has to be able to deliver to the large expectations created over the years. The risk of backlash is huge and there is no guarantee a new title will make a good enough profit to justify this.
As for Bethesda, @jedidia already explained. Game development aka making games has turned into a business, an industry, making it hard to compete or even survive in, most often ruled by publishers. Over the years, things have changed dramatically. There are huge risks involved, even more so when you are a big company, whether publisher or developer. Business models have changed, timelines to produce get ever shorter, investments required ever larger. You can not afford to risk anything, you have to act fast, invest a lot and get your return as soon as possible.
Once making money has become priority #1, it kills any passion, especially with so much (unfair) ‘competition’ out there, wanting part of the promised ‘gold’.
A Pink Floyd song keeps popping in my head … ‘Welcome to the Machine’.