If only that were true, I can only conclude that you must be new here.
Your right of course that a shit ton of websites have been single platform / single browser over the years. But by and large the majority of the web has been cross-platform, and the situation today is better than its ever been.
I think the web has been a great leveler platform wise and the situation has improved enormously, but I think the majority being cross-platform is relatively recent. Between 2002 and 2008 or so IE had a stranglehold and there are TONS of IE specific applications out there.
Right now with XP support disappearing the companies that invested heavily in Microsofts platform specific technologies, that Microsoft have now basically abandoned themselves. These companies are now scrambling to either migrate off XP and either find (crazy) ways to support their legacy apps or replace them entirely. (Yes Lloyds, I'm looking at you, you and your silly banking friends.)
On the upside, the're a royal fuckton of money to be made helping them.