There are a lot of people today to whom used is a bad term. Many of them have grown up in the "don't fix it, just buy another one" era of disposable everything. They are lost when considering a used anything, much less a rifle.
For them, the less expensive rifles work just fine. It's not like they will be out at the range daily.
For those of us who grew up during the days when things were expected to last, or have learned that durability still exists, used guns can be a bargain. Then again, most of the disposable era buyers don't seem to be able to sniff out deals, either.
I find it amusing to listen to how durable military arms are. MOST military arms have been used little, but carried a lot. Many have been refurbished, and then sat in storage for 60 years, sometimes more. Tell me how that indicated durability? It was a standard procedure to pull weapons out of the field at regular intervals in war-time, or even peace-time, and update and repair them as necessary. Try to find an as issued M1 Garand, Carbine, 1903 Springfield, or even an original M16 rifle today. Same with the weapons of the other military powers.
Sporting arms over the years have been built using the technology of the time. Back when everything was milled, forged, and hand-fitted, it was standard to make a gun that lasted several decades. Add to that the FACT that most of these guns were used in the same manner as today's. They were bought for deer season, sighted in, and saw action for a few days a year. People haven't changed that much in the past 100 years. They had jobs, troubles, and raised families. Hunting was often a seasonal sport, and the gun used varied with that season. Only the rich went to Africa, or traveled the United States collecting big game. Well, the rich and the gun writers.
Most of today's rifles are being bought by people with limited incomes, and limited time to spend afield. They might burn through 60 rounds a year. The less expensive rifles will last them both their, and their sons and grand-children's lives.