No, he just enjoys writing them so much. Here, let me give you the FULL list
The Colour of Magic
The Light Fantastic
Equal Rites
Mort
Sourcery
Wyrd Sisters
Pyramids
Guards! Guards!
Eric
Moving Pictures
Reaper Man
Witches Abroad
Small Gods
Lords and Ladies
Men at Arms
Soul Music
Interesting Times
Maskerade
Feet of Clay
Hogfather
Jingo
The Last Continent
Carpe Jugulum
The Fifth Elephant
The Truth
Thief of Time
Night Watch
Monstrous Regiment
Going Postal
The first of them was published in 1983. Only a few months after I was born, so he's been chugging these great works out for nearly 22 years. Terry Pratchett loves writing these books and will continue to do so for a long time. He publicly acknowledges the fact that he is rich, and that he has too much money to spend in his life. The only reason he writes is for the pleasure.
I hope I can get into a position like that someday.