No, they caught on because people obviously care about different things than you do.
See, not many people care if they need to download a 30k executable or a 100k javascript. They also don't care if that executable can perform whatever they need done in 0.2 seconds instead of two seconds for the web app. And they definitely don't care that you think that it's almost parasitic. Really? Why should they care about anything else than getting the job done in the most convenient way?
They care that they can get things done that they doesn't do very often, without having to find, install and run a piece of software locally - and risk getting malware at the same time or slowing down their computers.
Web apps are convenient. The user doesn't have to bog down his computer with tons of applications that he doesn't use very often and he doesn't have to worry (to the same degree) about malware. He will always have the latest version and he can access it no matter if he is at an Internet cafe in Thailand, at a friend's house or home at his desktop.
You may not like it, but web based apps definitely have several advantages over traditional, local apps and they do make sense a lot of time for a lot of people. Especially simple tools that may not be used very often, but also Office Suites (if you want to access your documents from everywhere), translation software or other software that constantly needs updating of data.
Sure, they may not - from a purely technical standpoint - be the most efficient applications. They may use up more bandwidth and total resources than local apps, but as long as they smoothly enough and are simple enough to use, noone except for the most pedantic programmers will care or even notice. They will just notice that it's simpler and takes less effort for them than downloading and installing a local app.