I think it is mostly that the flexible lines are easier and cheaper.
With the hard lines, you still need flexible hose at each end which is two more hose fittings per flexible line and two more hard line fittings per line, so with a return type system, and extra 4 hose fittings, and 4-hardline fittings.
In addition to that, it would be even more expensive for them to supply new hard lines for all the different vehicles. Most cars these aftermarket EFI systems are going on only have 5/16" hard line, and no return line, and on our old cars the old steel lines are likely full of rust and other trash that will get into the injectors. I like new stainless 3/8" hard lines, but it adds several hundred dollars more to go that route.