I think it's just fundamentally different cuisine. I was pretty baffled when I went in to a breakfast place and they had breakfast tacos with eggs, sausage and potatoes. I literally thought that was something McDonald's invented to seem "Mexican." The Mexican breakfasts I get have smoked beef tongue and stuff like that. There isn't really that much cheese either.
Another thing, and I'm not sure it makes a huge difference, but, the Mexicans I know up here tend to be from southern Mexico. The few Mexicans I know from further north say that the food in Chicago is mostly from Southern Mexico.
Tex-mex is pretty good, but if you are expecting Mexican food it's not really the same. It needs to be appreciated on it's own merits.
It's most related to northern Mexican cuisine, but pretty distant from that of, say, the Valley of Mexico or the Yucatan or Mexico's Pacific Coast, among others. Mexico has diverse regional cuisines, often influenced by the cuisines of the different pre-colonization local cultures (but also influenced by different post-colonization immigration patterns and other factors.)
Now, look at a Chicago Italian beef. Look at Pat's, Geno's, or Ray's in Philly. Look at a New Orleans Po' Boy and an East Coast grinder.
You think there's no regional variety within a country? Are you going to tell a Philly guy his cheesesteak isn't authentic because it's different from what you're used to?
Texas, California, Baja California, the Yucatan, central Mexico, southern Mexico, the northern end of the Gulf coast in Mexico, and north central Mexico are all very different places.
You're not missing anything, that is tex-mex and I promise you it is not worse. That is just how it looks but it's awesome.
Long story short: people's low-res cell phone photos of half-eaten food taken under fluorescent lights basically indicate nothing about the food.
[1] https://www.mcdonalds.com/content/dam/usa/documents/newbigma...
[2] http://louiseroserailton.com/wp-content/uploads/2016/07/mcdo...
[3] http://i.dailymail.co.uk/i/pix/2012/07/10/article-2171302-13...