Because whoever rote this was a psychiatrist. This is a classic tale of recovering from trauma induced psychosis, right down to Gulliver acting strangely when he gets home, to being put into the mental hospital, to having to face a panel of doctors in the end.
If it was just Gulliver's Travels, then the film would have a much different feel to it. But the film makers wanted to make a cohesive narrative that would seemingly connect with the audience.
Me, I could have done without the hole mental patient thing. Just thinking about it makes it really labored. Ted Dansen could have simply come home and told tales of his adventures to a bunch of kids or people willing to listen. Some of the locals might have then been curious, maybe summoned a doctor, at the end of which Gulliver reveals the lost sheep. End of story.
In fact, in my opinion, that probably would have been a better film. And if Gulliver had learned to buck social trend when he got home, then he could have done so quite normally, with comport, and stated his reasons during the course of the film, or at the end of the film. Heck, maybe even at the beginning to set the stage.
The more I think about it, the more I like that idea. Yeah, the whole recovering from mental illness thing was really unnecessary. The story of his travels could have worked without it.

Touche
The asylum only strengthens the thesis of the novel, highlighting the corruption and hypocrisy of the society Gulliver comes from, connected to its correspondents in the four voyages he has.

This wonderful adaptation was a distortion of the book for no other reason that to make it relevent to the time we live in,it highlighted the lone voice of sanity in the wilderness of belief in lies that governs our age. i honestly can not fault this wonderful film!

I believe this is better than the original, I thought the thing with this is what made this story better than the book.

GT was originally published in 4 segments, each book at a different time, hence Gulliver returns to England after each voyage. As a unified work, it gains in force by having the 4 voyages conflated into one.
The book emphasizes Gulliver's increasing estrangement from humanity, so the bit about the asylum seems like a reasonable addition. Swift had a lot to say about this subject in his other writings.

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You are myBelinda!

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