First off, I prefer quality over quantity. I'd rather play a 30 hour RPG with consistent high quality content than a 120 hour game with 90 hours of filler. Resources are finite, and the more developers spread themselves thin the more potential they waste.
Second, spending 2-3 months to complete a single game may not be a long time for you, but I reckon it is for most gamers that have jobs, huge backlogs, and maybe even other hobbies besides gaming. If every game I wanted to play was 100+ hours, I'd never get around to them all.
I understand your point about being able to skip content, but games are typically designed around experiencing all of the content. If I only play 40 hours of a 120 hour game, those 40 hours are guaranteed to leave me feeling unsatisfied. And again, the resources that went into the other 80 hours could of been used to enhanced the 40 that I played.
Finally, I'll reiterate again that replay value is more important to me than length. They're not mutually exclusive persay, but usually the more developers spend in length the more they sacrifice in terms of roleplaying options, quest design, and reactivity. The longer the game is, the less likely I am to replay and try different paths as well.