I see in pictures from that era that men - even unemployed men in bread lines - were always wearing perfectly tailored suits. How is that possible, given the expense of even really cheap suits now? How was affording so many suits within their means before garment mass-manufacturing? Were tailors a-dime-a-dozen?
Firstly, people didn't tend to own as many clothes then as they do today. I'm not particularly up on early 20th century fashion trends as 18th-19th, but during the latter period, many working class people seem to have owned only one or two coats, but slightly more in the way of vests, trousers, and shirts. For many people during this time, one suit of clothes would be worn on an everyday basis, and a newer, nicer suit would be reserved for special occasions.
Secondly, exactly because mass production wasn't so advanced, the gulf between fairly nice clothes and cheap clothes wasn't as broad as it is today. Many of the innovations (fusing instead of canvassing) that allow bargain suits to be manufactured cheaply simply didn't exist. Suits made in the US or Europe today using the traditional methods generally retail for $1000 or more. This is to say nothing of the lack of globalization. The American garment industry in the early 1900s was fairly dominated by unions, ensuring that rock-bottom low wage labor, so widely used today in various developing nations, was not really present. Because of the higher wages, garment workers tended to be more skilled than they are today.
Because of these factors, while people owned less clothing, and paid more for it, it tended to be of significantly higher quality than what most of us, myself included, have in our closets today.
I have a related question: Why did people stop wearing suits as everyday wear? I would imagine comfort being an issue, but then why did this fashion start in the first place?
In the 18th century, a working class person would own only one coat; working class people in general owned one outfit of outerwear that they wore every day. Men wore a shirt underneath that they would wash, as washing the outer garments would wear them out. Middle class people might have two or three outfits. Tailors weren't a dime a dozen but labour was extremely cheap. The real cost was the fabric, so if you used cheap fabric the suit would be cheap as well. However in the 19th century, fabric was being mass produced so poor people might be able to afford another piece of clothing if they're not destitute.
As to why they could afford suits - apart from wokring wear such as overalls, the suit was all that was available. There wasn't anything else to choose from.
And in general until the mass manufacture of garments, everything was made for the individual. There was no standard sizing (or even a standard system of measuring someone) or stores full of pre-made garments to choose from. You could buy something used that more or less fit, but you would at least alter it. Clothing fit much more closely to the body, so it would be more apparent if it fit incorrectly; and it might be your only outfit as well.