Because Prowlers are fucking ridiculously expensive to buy. Especially when it costs little more than a pittance to make them if you know what you are doing.
Example: I got my prowler for 70euro of a friend who knew how to weld. Cheapest price I can find for one locally is 220euro. And to get one shipped from the states(there were no local suppliers back when I got mine) would have cost 700fucking euro.
OP, somebody on here posted these dimensions a while back. I can't remember who it was but since they posted it publicly I doubt they'll mind my reposting of them.