It's very situational for me. Most of the time I would be just as happy with Maxis lots (though if they can outsource some of that work, and spend more time on gameplay, that's a plus). For Cottage Living, I thought it was great that they picked folks who are actually English to do the builds, and it really showed in the authenticity of the builds. I've kept every single one of those lots with only minor decor and landscaping changes. (Even though I wish we had a few more empty lots for Henford.) I wish they had done the same with Snowy Escape, honestly. For Growing Together, I think it's fine that they picked Simsie, because suburban craftsman homes are right in her wheelhouse.
But in general, yes, I think it would be great to branch out to some other builders. That being said, whenever people cite examples of other builders they would like to see chosen, their work typically seems to be... beautiful and interesting, but also, doing the most. I.e., just way too cluttered and crowded and busy to be functional for gameplay for most simmers.
Also, putting on my lawyer hat and my supervisor hat for a moment, I can see where the devs would want to go with a known quantity. When they are dealing with the same handful of people over and over, they know that they can quickly update the same old contract and get coroporate approval easily, they know that those builders are likely to actually abide by the terms of those contracts (confidentiality, timely completion for deadlines, etc.), and they know that those builders can follow instructions and take direction. Also, let's not underestimate the importance of being pleasant to work with: if you have to choose between someone you know will respond pleasantly and professionally to feedback, someone you know probably won't, and someone for whom you have absolutely no idea, most people are going to choose the first option.
