Because many birders form part of a family, and they don't want to be left at home
There's no reason why you shouldn't bring your family too. If they don't share your interest in birds and don't want to participate in a birding tour then they can be directed to interesting sites of tourism interest nearby, or could even be persuaded to take part in some novel and exciting activities. In other words birding or…ballooning, rafting, cycling, wining and dining, etc.
Port Aventura, Costa Brava and Costa Daurada, trekking, rafting, cycling and horse-riding in the Pyrenees, Boí Valley world Heritage churches, Gaudí's Sagrada Familia, canoes and pleasure boats, museums, castles, cathedrals, towns and villages stepped in history, water sports, skiing, world famous olive oil and wine, sunshine, amazing scenery, boutiques, friendly family accommodation...
"Hey kids, want to go to Port Aventura in Spain this summer?" (Birding thought: Red-necked nightjars breed in the grounds).
"We could go ballooning over Montsec." (Birding thought: Lammergeiers are regularly seen in that part of northeast Spain).
"We can do a bit of trekking in the Pyrenees in the morning and then rafting in the afternoon to cool off." (Birding thought: And I could spot Citril Finch and Black Woodpecker in the Pyrenees).
"The views from Mont-roig are breathtaking!" (Birding thought: And according to Where the birds are in northeast Spain Mont-roig is a reliable site for Black Wheatear and Blue Rock Thrush).
"Darling, in the Ebro valley area wines of international renown are produced, like Somontano or Raïmat." (Birding thought: With a bit of luck we could get lost and bump into a Little Bustard or two).
And then you can always round off the day with a paella, or fresh fish washed down with an excellent wine....and make your birding…er….holiday plans for the next day. Those are just a few examples of what you can do. For more detailed information about the activities that can be done check the must visit section of the birding itinerary you're going to do.
If what you're looking for is a reason to leave your family behind, sorry but you won't find it here!