Because they're blithering idiots who confuse "can make a contact" with working properly. Well I can make a contact using a 50 Ohm dummy load as an antenna but that doesn't mean its any good and I can make contacts if I pull the PL259 out of the antenna socket so the centre is just making contact but that doesn't mean its working properly.
There is absolute no way in hell a magmount has enough capacitive coupling for 900W and the coax they come with certainly isn't rated for 900W. That 900W needs to return to earth and will be going somewhere and that somewhere will be right back down the outside of the coax meaning the gear you're using will be hot with RF with the potential of RF burns (you don't feel it burning as it is burning you BTW) and it'll be getting into the vehicle wiring and causing all kinds of issues.
Don't believe me? Run a long length of coax from the magmount, get a flourescent tube and whilst someone is keying the rig, put the tube near the coax. Watch how bright it lights up. Its being powered by the RF current flowing on the outside of the coax.
There is absolutely no need for more than 100W. I had a Kenwood TS480HX in my car which is 200W. I replaced it for a Kenwood TS480SAT which is 100W and noticed absolutely no difference in the contacts I could make. I was still able to get to Australia from the UK and I still had stations in the USA giving me 5/9+ signal reports and being astounded I was mobile.
All having 900-1000W in a mobile install is doing is satisfying some penis deficiency. It serves absolutely no useful purpose.
