That might be a valid argument if you ignore the fact that 95% of transactions go through without any problems. With MTM tailoring you're always going to get some issues. There is a lot of scope for human error to creep in. Orders not being processed properly, tailors making mistakes and of course being given wrong measurements to start with.
However, I get far more emails every week from customers thanking me for their great fitting suit than I do from customers with problems and when you consider only a minority of satisfied customers will email you to say thanks as opposed to every customer that has an issue will email you it's a good indication that we're doing things right. It's the same with forums, people will rush to the keyboard if they have a problem with their suit but rarely will you see someone post something like "have a look at my great fitting suit".
Take the Indochino thread here for instance. Hundreds of pages of, in many cases, not very good fitting suits. Yet Indochino sell thousands of suits every month yet we only get to see say ten of them and mostly they have been posted because there are issues with the fit. Do you think Indochino would have survived if they couldn't make a decent suit much less grown into the behemoth that they are now? We rely on repeat business to continue to grow. Would customers come back for more suits if their first one wasn't satisfactory?
What's the alternative? Buying off the rack? Unless you're extremely lucky your OTR suit is going to need alteration. Yet the pictures we see in all the MTM threads here are almost always as the suit was delivered. If it's your first MTM suit there is a fair chance it will need some tweaking after it arrives. Bring in the jacket waist and hips a touch, a centimetre off the sleeve or jacket length. Once you have that done to the first suit you then have a template for the second. Tweak the measurements again for the third and so on until you get to the point where you can simply send in a fabric code and say make it the same as the last one.
If you want an OTR fitting you can have one. We will send you a size 42 jacket and 40 pants (for example) in a hand made fully canvassed jacket and made from some fairly decent wool for only $399. You will look long and hard to find anything of similar quality in a store at all much less for that price. For only $999 we will send you a completely hand tailored suit made from top quality English fabric such as Holland & Sherry in a standard off the rack size. Good luck finding that in a store.
Store bought suits are almost always a cheap factory made fused (or if you're very lucky half) canvas suits slapped together by machines and made from the cheapest wool they can find. No matter what you do to a crap suit it will always remain a crap suit. A well made full canvas suit made from good wool, even if you have to have it altered, will look better and wear longer and prove to be far better value over the long term than any cheap factory made fused suit