It's a slow night and I have nothing else going on... so might as well blather a bit.
There may be a secondary issue with this "idea", besides turbocad's clearly (budgeting and saving... it's a new concept to some) defined economic one ... and that is fitment and alignment.
A good quality mech will have all it's tubes hand checked for fitment and function prior to final assembly and packaging. It may even be tested for voltage drop.
If something doesn't fit right or look right, another part is pulled and checked until you have a complete mech. There is always a certain degree of corrective action (aka "re-work") required when you're dealing with several hundred of a machined thing. Are you willing to substitute yourself in that role... and then what, if one of your parts is not right?
Next is engravings/laser etchings. With some mechs, a logo, a brand and perhaps a SN my be on separate tubes... but amazingly, they all line up, or are 180 degrees opposite one another. It's not through intense analysis of thread pitch and clocking skill... it's simply that the engravings are applied to an assembled mech. Buying in pieces means... a 1 in 360 degree chance of perfect alignment.
The same can be said for hand polishing and special finishes... especially a brushed finish, where the brush (or even the polish - under magnification) on one tube is a perfect match for it's mating tubes, if any.