Board Thread:Lore Discussion/@comment-13417807-20130726023954/@comment-1470368-20150213224009

2 Main Reasons:

1.)  Most people who've only been with the TES series since Skyrim have either ignored or failed to notice any examples of decent Altmer.  It is also possible that they've purposely blinded themselves to any such examples after encountering just a few Thalmor (and have thus mentally linked the words "Thalmor" and "Altmer" as being "basically the same").

2.)  Some people, even a few who've been with the series since at least TESIII: Morrowind, read too much into the Lore and take it for granted...somehow believing it to be "100% accurate" (including established stereotypes about the Altmer).  Some of these people like also believe the Pocket Guide to the Empire (1st Edition) to be word-of-god (And not Imperial propaganda like MK himself even stated it to be. He's the one who wrote the Pocket Guide, after all).