Because it was in an earlier version of the script and never got removed.
The Truman show wasn't originally set in a seaside town, it was originally set in New York. Making Truman's father "Han" a homeless man made good sense and when the locale changed, it would appear that they simply decided to keep that part of the script, noting that right up to the final edit, Truman's world was significantly less utopian.
Pausing to check his profile in the mirrored building, he glimpses the
reflection of a HOMELESS MAN standing directly behind him. Truman,
spellbound by the man, suddenly wheels around to face him. The
Homeless Han is in his late-sixties. more well-groomed and well-fed
than the average vagrant, with a serene smile on his face.
Truman Show: New York Script
In a deleted scene from the film, Truman's suspicions were aroused when he saw a 'homeless extra' in a wheelchair who was ill-advisedly recast as a 'jogging extra', so it would seem that homeless people did exist in Seahaven, at least before the edit was made. The scene also existed in the earlier script.
A TRANSIENT in a wheelchair approaches, looking for a handout. Truman gives the homeless man half of his sandwich, reconsiders and gives him it all, his appetite gone. As the transient wheels himself away, Truman loses himself in his book.
Truman Show: New York Script
For the record, the earlier script is abundantly clear that appearing as a homeless man (complete with makeup) was a disguise, not how Kirk/Walter/Han normally looks.
At the only occupied table, Truman's contrite father, KIRK, is having what's left of his homeless disguise cleaned from his face by a MAKE-UP ARTIST under the watchful eye of two DARK-SUITED BODYGUARDS.