To MYOG, or, more broadly, to do anything yourself comes down to opportunity cost. Both in dollars and in the case of UL gear, in grams.
As you pointed out sometimes with sales and coupons and what-not one can find a good price even at REI, but I'm not sure one will typically find gear that is as lightweight as what one can MYOG. Buying gear that is as light as a homemade cuben anorak is pricey and, although very rarely especially now that there are so many fantastic cottage manufacturers, maybe not on the market.
For me and I imagine a few (but only a few) others, there's the angle in that I'm not comfortable with purchasing a piece of gear, say a shoe, that was made by a brown or otherwise non-white child somewhere in the Global South. A child that was paid a few pennies an hour and was worked for 60+ hours a week in unsafe conditions. And then the Western trans-national conglomerate spends more on the marketing of the shoe than on the raw materials or labor.
There's another angle though and that is: how much value do you personally put on that one thing you're not going to find anywhere else? That is, how much is the satisfaction and experience of doing it yourself worth to you? Judging by the amount of awesome gear made by Bill Fornsell and Newton, just to name two, it's about much more than necessity or thrift. From just the posts of theirs I have seen, they each must have several closets full of gear they made themselves. Some of it quite innovative and still not on the market after many years. I don't think they made all that gear because it was "needed," but because part of their long-distance walking hobby is the creation of gear and they find a satisfaction in that aspect alone.
Jamie summed it up nicely, I think.