If you use sectional frames you can buy pairs of sections of assorted and sundry lengths, usually in one or two inch increments. With two such pairs you can make about any aspect ratio you want. I occasionally do that for a particular use. Outfits like Frame Dimensions, an APUG sponsor, will supply about any combination with matching glazing.
Custom framing is easy enough to do, but it still isn't as easy or as quick as as unwrapping a ready made matted frame and popping a print in. They're merely a convenience thing to be able to skip fiddling with them. And I kind of like the convenience of being able to walk into a store and pick up a handful of frames to use that afternoon. Less mucking about or having to wait for an order to arrive. As it is I have a shelf in a closet taken up completely by frames I've ordered, and that space could totally be put to better use storing more cameras or something.

Honestly I find the most annoying part of making custom frames myself is doing the mats, but that is easily solved with eventually getting the space and budget for a far better mat cutter jig. For me that is a fairly minor thing that will get solved eventually after a move into a new place. (Work spaces are kind of at the top of the list for the next place I get into.) Paying someone else to do the frames for you isn't really all that annoying, but tends to be rather painful on the wallet.