I hear what you say, but believe me, you'd have a hard time getting a consistent result doing it this way, good enough for personal use but not for a commercial product. And this comes from a guy who loves messing about with bits of metal
Mild steel and aluminium are easy to shape, for sure, but they hardly represent an improvement on your bog standard zinc alloy Chinese bridge. If I were to make or buy a 'better' bridge (ie. more durable and/or different tonally speaking), I'd want it to be stainless steel. You can work SS by hand and with simple tools, but it takes more time, just because it's so hard and takes a few tricks to do it right. And we're just talking about the U-channel here - which by the way is harder to source in the right dimensions and alloys. You still need a way to attach the right posts underneath, and unless you find just the right short rods with internal threading, you're in for a lot of pain drilling & tapping your way through 12mm of metal, keeping things dead straight & centered. The other thing is, your tools are going to wear down much faster. Files, taps, drill bits, sanding paper, etc. are a budget in themselves when you do this sort of stuff full time.
Once again, I'm not saying this can't be done. The masochist it me even enjoys doing this sort of stuff. I find elation in getting the speed & feed just right or filing a perfect radius, but having made a few bridges (and saddles, I think I posted something here about it yesterday), I got to the conclusion that unless I needed something very specific and exotic, paying £60 for a Staytrem bridge was a lot cheaper in terms of my own time and labour than making a bridge from scratch. If I boil things down to an hourly rate, it's just not worth it.
Obviously, both the Mastery & Staytrem offers are CNC machined and it doesn't take nearly as long as handcrafting it, but if you take the production costs into account (and those come with their own saddles), they are not that pricey when think of it. All those Chinese eBay/Aliexpress bridges are cast, using inferior metal alloys. Still harder than your mild steel/aluminium from the hardware store, but more likely to fail in the long run than SS (even though mechanical failure is hardly ever the issue with those parts, something that will last a life time sounds better in the long run on our cherished and over-abused axes). The Fender stock bridges are also cast, and may or may not use better materials.
To complicate things further, I find that the density of the materials used in the bridge are one of the key factors in the sound of a guitar. (I'm getting into a slippery territory here and speak strictly for myself here, all things tonal being a highly personal thing
). There are instances when a lighter bridge is more desirable. I made a stupidly lightweight aluminium bridge for one of my builds to get a shallow but springy, clicky tone. It works great but I know it'll wear down really quickly. So in a way, those cheapo internet bridges have their place in terms of guitar-making - they do do things a heavy stainless steel bridge can't do, but it is quite incidental.
Sorry for going off-topic here, I realize I spend much more of my waking time thinking about these things than I'd like care to