The cost of attending sporting events have skyrocketed over the years, so that for the most part, attending sporting events for those that make a modest income or less, is something that for professional sports has either been eliminated entirely, or curtailed significantly, except if attending as a guest or on those infrequent nights when special pricing is in effect or no-interest has been generated in which case, list ticket prices are discounted significantly.
One of the peripheral things adding to the expense of seeing live sporting events is that all stadiums with slight variations in their rules, want to be the master of what you eat or drink once you enter within the stadium. That is to say, extra income is generated for sport franchises, from the buying of food items, refreshments, and alcoholic beverages, and the stadium obviously prefers not to have to compete with outside elements.
In actual fact, although not well advertised because they have no interest in cannibalizing their own sales, most stadiums allow patrons to bring in plastic bottles of water or plastic bottles of non-alcoholic beverages, as well as most stadiums do allow you to bring in outside food, as long as it is contained within a visible plastic bag or a soft-sided bag that conforms to the size limitations of a given stadium.
On the other hand, no stadium legally allows its patrons to bring in alcohol of any type, yet, without exception, all stadiums sell alcoholic beverages as long as you are not obviously intoxicated. This would indicate a general hypocrisy in the sense that alcohol is permitted within the stadium, as long as you buy the alcohol from stadium vendors, but you cannot bring in from outside your own alcohol, which means, basically, the stadium policy against outside alcohol, is essentially to compel its patrons to buy their alcohol from designated stadium sellers. I supposed, to a great extent, this makes sense, especially since you are allowed to fill up on your own alcoholic beverages in the parking lot before entering the game, still, for people on a tight budget, this might encourage them to drink more than they would prefer in the parking lot, as opposed to a more consistent and reasonable pace if they were allowed to bring in one or two alcoholic beverages contained within a sealed plastic container.
That said, the biggest boon to stadiums and their concessions has got to be airport security which has pretty much trained people that you can't bring any liquids successfully pass security and that food items are also pretty problematic, even though you are allowed to bring in snack items, or even sandwiches or burgers if they are wrapped in a container or sealed in a clear plastic bag through airport security.
Quite obviously, stadiums don't make it a policy to explicitly advertise or encourage you as to what you can take into a stadium, and pretty much prefer that you assume that the correct answer is no liquids and no food items. However, to their credit, and especially to their credit on behalf of families, their policies are reasonable and will thereby allow a family of four to attend a sporting event, without having to unnecessarily worry that they must also eat or drink only stadium provided foods, therefore most definitely saving that family some meaningful money, and making it more probable that the tradition of attending sporting events can successfully transition from one generation to the next.