Yes because it would make the company more money
Not necessarily as it would require a liquor license and new training for employees, and since people don't tend to go to McDonald's seeking alcoholic beverages wouldn't necessarily generate enough sales to increase revenue to cover those increased expenses.
You only need to pay for those things once. And the training is not extensive, it's literally just telling them they have to card anyone trying to buy it who looks under 50 or whatever.
Actually, a bigger problem for McDonald's would be that every state has a minimum age of at least 18 to serve alcohol, with many having it higher, especially for liquor.