Not "sad" at all and frankly if I were your friend and that were your attitude I wouldn't bother making any for you in future. What an ungrateful person you are!
There are loads of reasons to do this.
To avoid overpaying for poor quality food as is often the case in tourist attractions, plus not everyone can afford travel, admission fee AND overpriced shit food.
To avoid long queues or having to trek to where the food stands are
To take food you know you'll enjoy, especially true if you have allergies or other dietary requirements which can limit your choices away from home. I'm vegetarian plus I have stomach issues which mean I don't tolerate very spicy or certain high fibre foods well. It gets very boring only have veggie burgers (often "spicy bean burgers" or crap cheese or egg sandwiches as the only veggie food available), dd is pretty unfussy but has never liked chips or chocolate (now know these are irritants for her probably due to her disability). Other people might be on medications that mean they can't eat certain foods or be diabetic or have high bp which means their diet needs careful management.
To not sabotage a weight loss diet. Often in these places it's "chips with everything" or half a ton of mayo on a lacklustre sandwich!
To not be restricted by opening times of the food services.
I've also had the very unpleasant experience of food poisoning from the type of places selling food at these places. They're often being run by less than circumspect late teens/young adults who are being paid sod all and rushed on their few breaks so skip washing hands as frequently as they should or observing food hygiene regs (yes I know they have the qualifications and know what they SHOULD be doing but I've frequently observed them NOT doing what they should)
"Doesn't anyone else find it a PITA to cart it around all day with you?" How much do you eat?! When on days out of this type dd has carried her own for many years and me too we take a wee rucksack each which we fill with lunch, snacks, refillable water bottles, sun cream, plasters, painkillers, wet wipes, power pack for phones, purse, phone and keys.
Not particularly heavy or cumbersome to carry and cover most eventualities.
So yea yabu but kudos for acknowledging it. Hopefully you'll be more thoughtful and grateful in future.