Why do peope get offended if I say I don't like Mexican food ?

Mexican and Spanish people/food are not the same...not even close. The only thing they really share, is the Spanish language, and even that differs from Spain to Mexico.

As far as people getting offended because you don't like Mexican food? That's their problem. People are entitled to like what they like. Not everybody likes all ethnic foods. I for instance, do not like Indian food.

However, you might be mistaking people's surprised reaction as offense. You didn't mention if the "offended" people were Mexican immigrants, Mexican Americans or non Hispanic Americans. Mexican food is the second most popular ethnic food in the United States. Chinese is number one, and Italian is number three. People might just be surprised when you say that you don't like it since so many other people do.

You should be aware though, that not all Mexican food is the same. It differs from region to region and state to state. It's not all just tacos and burritos.

Mexicans are always trying to prove they're worth something. They see all rejection as "nope, you still haven't produced anything valuable yet"

Authentic Mexican food is really quite good in my opinion. You can taste the Greeco-roman / Mediterranean influences. (So much so, it sometimes makes me wonder if they just renamed some forgotten dish)

I guess, they're on the defense. It doesn't have to be rational, necessarily. Culture, traditions, customs, etc, just hold personal for many people.

Anyway, who needs Mexican food, when we have Asian food?

They confuse "I DON'T LIKE this" with "I HATE this", even though it is possible to tell what is "lack of preference" from "hatred". That is being too sensitive about words, or maybe they were victims of some racist incident or bullying.

Likewise, it is possible to not like the colour gold, without having to destroy everything that is made of gold or with a golden color.

They want you to eat that battery acid covered slop. Mexicans were all stone age Aztec cannibals without the wheel or metal just 500 years ago. Mexicans are still as sub-normal as their recipes.

People take things too personally. If you say you don't like their food, they think it means you don't like them. People are so quick to take things personally and see everything as a personal attack on them. Just explain that the taste and textures of the food are not your preference.

Also, Mexican/Spanish are not the same thing. Mexico and Spain have different people and cultures. Spanish people are white Europeans, and Mexicans are generally Native American or predominantly Native American. Their foods are totally different too. I'm not sure why you would say Mexican/Spanish people and food as if they were the same. They're not... at all. And you should know that since you're European.

You just said it and I'm not offended, so your theory needs work.

Because you're actually attacking, largely, the beef and dairy industries that are powerhouse production industries in America.
Essentially, you're speaking traitor talk against the GDP.