you know the answer already, but ill play along.
it has nothing to do with cost. for what i am willing to pay for things (and im no baller), either option could be more or less expensive.
imo, there are always patterns and fabrics that are just not available outside of RTW. sure, you can find a navy fabric or a check or stripe anywhere, but there are always patterns that just aren't found in swatches.
for example, i really wanted a GP/PoW suit. initially i wanted to get one from panta. he sent me a ton of swatch books, but in the end, none of them spoke to me. then when NMWA opened up they offered the formosa GP/PoW suit that was exactly what i was looking for. the right fabric texture, the right colorway and the right size plaid scale. it was exactly what i wanted. the pattern was cut by formosa from LP fabric.
is it possible that if i scoured every fabric book out there that i could have found that or something like it? maybe yes maybe no. its possible they bought those rolls exclusively, who knows. either way, there will always be items that are available to buy RTW that one wont find in bespoke/custom.
of course the same is true visa versa. i had ed make me 2 suits (one is still in the works) and i dont think i ever would have found those 2 RTW. they were not standard colors and fabrics, and i doubt i would have found anything quite like it in a store. online or B&M.
then there is fit. sure, its nice to have something cut for your body, and yes some people have body types that apparently just dont jive with RTW (i really believe such people are the large minority. with a good tailor most people are more than fine with RTW). still, there are many cuts that can be made, and not every house will make every cut, and if you ask them to veer from the house cut, there is no guarantee that you will like it.
lets go back to formosa for example. they have pretty extreme cutaway quarters, and a very prominent X shape silhouette. in a million years i never would have thought to ask for that, and i would never have thought that it would look as good on me as it does. not to mention, there is a very small likelihood that anyone else would quite replicate that. every house has a cut they do best, and veering from that is often asking for trouble.
again, on the other hand, i love the cut of the suits ed and tiberias made for me, and its unlikely i would find that in RTW either.
certainly you have the best shot of the best fit with bespoke, but tbh, i find that some of the custom stuff i see here, i dont like it. now, thats not my business, it matters that the customer is happy. but going bespoke is no guarantee that you will like the fit/cut, no matter who you use. some RTW brands make cuts that i dont see in bepoke, and of course, many bespoke things fit better and nicer than RTW.
for someone like me, who can find RTW that looks great (imo the RTW i have looks great, now that i have hones in on my size and measurements, and can properly direct my tailor what adjustments to make), i see no reason to abandon RTW. and more than that, i really feel it would be a net loss for me to exclusively use one over the other. they both provide good options for me, and its just a mater of navigating properly and getting what i need and want from each.
more options = better, imho.
tie is BB 100% wool.