People play flash games because they're at work/school/home on their average computer of average age and the games don't require a dedicated GPU, which is generally what they're made for. Even a lot of indie games run great on these systems, being popularized by Steam and Humble Bundles. Even then, these average computers can still do well in current games by simply throwing in a GPU and some extra memory.
imho, people generally haven't had a genuine "wtf my computer is slow" feeling since the Pentium 4 HT / Athlon 64 days. Anything Core 2 Duo or better still feels quick by today's standards and, as others have mentioned, run the latest operating systems rather quickly. Heck, my dad was happy with his 1.6 GHz Socket A Duron running Windows 7 for the longest time. I got him a computer to replace it (one that someone else was throwing out because it was running slowly), but he didn't switch until his motherboard died.
While volunteering at an electronics recycler a while back, we'd have people come in saying they had no money and that they needed a PC to help them write resumes so that they could get back on their feet. We'd say "Sure! How about this 2.4 GHz Pentium 4 with 512 MB of ram and Windows XP?" and they'd go "Oh, no, that's okay. Thanks though." and leave, the "higher end" stuff being socket 775 P4s.
Unfortunately, the only time I've seen an average user throw out a PC is when they've royally messed up the OS with either malware or system "optimizers". They take it to Geek Squad (salesmen in geek's clothing) who instantly tells them it can't be fixed and directs them to their showroom floor. I'm sure plenty of us have fixed tons of "unfixable" computers by mashing F12 and booting to the recovery partition. In this sense, we are contributing to this problem.
I guess the main issue is we've hit that "fast enough" speed a while back. As others have said, the big thing now seems to be tablets, both in the usability sense and in the "we don't have the technology to make these things 'fast enough' yet" sense. I suspect we'll be seeing a similar "too slow, throw it out" trend with tablets in which case these manufacturers need to transition from PCs to tablets, similar to the transition from carriage to car.
tl;dr, manufacturers need to adapt to survive and stop blaming us for not buying in.
