So on the other hand, then why do we estimate (with extremely limited information) that this project will take 7 months for 8 developers, and exactly TO THE DAY

So on the other hand, then why do we estimate (with extremely limited information) that this project will take 7 months for 8 developers, and exactly TO THE DAY, we deliver the final projects 7 months later and the client is happy? AKA you can't estimate pieces, but you can still say this will be done in 3 months, and then you manage scope etc. so it's done in 3 months. Or: you can't estimate how long this will take, but you can set a "how long" given flexibility in the "this".