Because unless the chassis is anything other than 100% bare, clean metal on the INSIDE of the box section (which is highly unlikely on an old chassis) the galv process won't take to it, will remain bare metal and will therefore eventually rot from the inside out, so may as well just wire wheel down the outside and cold galv paint it to get what would essentially be the same end result.
It happened to a customer (who happened to be there with him at the point I was talking to him) whereby he paid somebody £3000 to supply, hot-dip galv and fit a second hand chassis. X amount of time down the line, aforementioned rotting from inside out then occurred and ended up going to Richard Chassis for a brand new, properly made one which cost him another 3k+.
For £1600 plus the VAT, he'll make up a brand new, made from scratch, galv'd D1 chassis for anyone interested.