I can think of potentially two reasons:
1. People think Pep will opt for the "element of surprise" as doubtless Swansea will be expecting the same lineup, given it's been so successful. This doesn't seem to be a legitimate argument in modern football and one need look no further than Chelsea for evidence.
2. People think Aguero/Dinho should play regardless of form of other players or the team as a whole, either for sentimental reasons or otherwise. I don't think City are in any position, particularly with top four competitors dropping points again, to go along with this.
Regardless, I happen to agree with you for the most part. It's very possible Pep will change things up, but it shouldn't be something people are actively searching for. We've stumbled from one lineup to the next and lacked the consistency that comes with stability. Finally we have what looks like a genuine threat, a team that actually appears to be clicking according to Pep's unique and demanding, but incredibly rewarding, style. Why would anyone want to roll the dice on another lineup?
I dont think Aguero/Fernandhino should play every game regardless. But I do think they have a major part to play in the remainder of our season. And this seems like the ideal opportunity to re-introduce them.