I agree taht with 4 people, why not go on your own, rent a car, be on your own schedule. I"m sure you could do a trip for much less than the tours.
Because cost is not always the only consideration.
For example, a lot of times, one person ends up making all the arrangements, doing all the work, making all the decisions, taking care of everything and everyone, and that person never gets a chance to really enjoy the vacation. Having someone (like Disney) do it for you means everyone gets to enjoy the vacation & relax & have fun.
Also, not everyone is comfortable going to a foreign country, where they do not know the language or the currency or the customs, and trying to coordinate things themselves. It's easier if you let someone who knows the territory do it for you.
Or you may not like standing in line. A lot of the places ABD goes in Europe, they get to skip the lines that the people who "did it themselves" have to stand in for hours before they get entry.
Or you might like the security of traveling in a group, and having someone looking out for you, rather than strictly looking out for yourselves.
Just a few examples of "why".
If cost is your only criteria, then yes, you could do it cheaper. If you are concerned with other aspects of traveling, then "cheaper" is not always the deciding factor.
Sayhello