Earlier quoted context omitted.
Because having to support a lowest common denominator of slow and feature poor browsers is a nightmare.
That "nightmare" you are bitching about is called either "your audience" or "your customers". I am not that all that web savvy, but I try to keep things accessible, both in terms of systems used and in terms of physical limitations. My two main websites are aimed at folks who tend to have physical limitations and thus also financial limitations, which means they rarely have the latest, greatest tech. So perhaps my vi…
Building a website allows for little in the way of excuses as to why you shouldn't be able to support almost every single user in every single browser.
Building a web application allows for a much greater variety of excuses. Writing modern code that allows for dynamic interaction and 'save as you type' type of functionality is extremely difficult to support for older browsers or browsers that don't do a very good job of conforming to web specifications.
You've mentioned previously that you aren't a 'developer', so I thought it merited mention. Simply put, it's a lot harder than you think.
If your app is in a competitive space, or one where 'cutting edge' is expected, it's an extremely easy decision to throw away less than 1% of your audience in favor of being able to iterate faster, release new features earlier than your competition, or dramatically increase the speed in which the other 99.7% of users interact with your application (and make them happier customers.)
I generally agree with the notion of 'supporting everybody you can', but having built 'new-school' applications, I understand that it is in fact very hard, and not always worth it.