Because the rule is insane as listed and requires extra bookkeeping of "original days" and "extra days" How incredibly incompetent do your workers have to be to actively set back the construction time of something? "I'm leaving for a day. Don't touch anything! When I come back, we'll only have two days of work whereas if you idiots do anything I'll have four".
It's so crazy, I'm not certain it's RAI even if it's RAW.
It makes more sense if you imagine the actual project taking 3x the time, and the presence of the PC just speeds things up by that much.
Like, a 60 day project is what it is from the PC's perspective. From the perspective of the rest of the world, it's a 180 day project that the PC can manage to get done in 60 days.
Because this is 2014 and our idea of heroism now extends to management principles like efficiency. "I will heroically lead this team to beat our bottom line projections! I took the Agile Development feat and multiclassed into Scrumlord for the +2 to Effectiveness from Standing Meetings feature."
Papers & Paychecks, you're getting closer every day.