This is a noble concept, and sometimes it is the reality for business critical software. For consumer products, this is almost never the case. Microsoft tried this with Windows for many years, the bloat grows exponentially with each release.

For a consumer product it is all about tradeoffs. Most consumers like a simple and intuitive feel (which changes with every generation). Interest drops and frustration quickly increases with additional options. Very few users are experts in the product, we can't all be experts at everything all the time!

Then there is also a psychology layer to entice the kind of consumer behavior that aligns with the business model. Too many options prevents a company from steering its users in the desired direction.

Business travelers usually have their own portals/services/TAs. Consumers want the lowest price and simplest experience. Expert travelers using UA.com may not be a market UA wants to support. If we are headed toward a commoditized travel market where price + (ambiguous marketing effects) is king, extra features that don't monetize easily are dead weight.

Many other factors, all a tradeoff/compromise. Can't please everyone.