probably because most manufacturers use flywheel figures when describing thier engine which is likely to go into a number of vehicles/transmissions.

an example, rs2000 fwd and 4wd have same power output from the engine but the power at the wheels is less on the 4wd due to transmission drag etc. How do you explain that as a salesman to your average punter??? If your qouting engine fly power then you just say its slightly slower due to 4wd and the driver nods. If your trying to explain why one puts out 130bhp at wheels and the other 115bhp, yet has same engine then you need to explain about transmission losses etc.

Plus obviously from a sales perspective 150bhp sounds better than 130