IE 7 came out in… 2006. So IE 6 just sat there for 5 years. And it’s not like ever upgraded to 7 immediately. Lots of corporate PCs kept 6 for many years longer.
That’s why it has the reputation it does. By 2006 IE 6 was horribly behind FF by every measure. Tons of security bugs had been found. By 2010 it was a total joke but developers were still having to make the modern web work on it, somehow.
If it had been replaced in 2003 or 2004 and MS never paused development no one would remember IE 6 as a dumpster fire.
But once you get a total stranglehold on the market, you can do stuff like that.