Neither did Boba Fett as a little kid.
Actually that reason alone is good enough for a Boba Fett standalone movie. Lots and lots of spinoffs can effectively be a papering over of the prequels.
What is it with people where they feel the need to have background movies/shows for every semi-popular or popular character in a movie or show? One of my biggest pet peeves in Hollywood is when a popular movie or show gets some sort of prequel, it seems that producers feel the need to insert younger versions of even lame characters around every corner. Many of these characters (such as Fett, who I'll discuss later) are popular precisely because of their mysterious past and origins. Expounding on these folks' backgrounds should be left to fan fiction or the lame EU-type material.
Boba Fett isn't getting a standalone movie because he was a snotty little brat in the prequels. In fact, his appearance in the prequels (or, more specifically, his portrayal in the prequels) diminishes him in my opinion. He is getting one because he has been one of the most mysterious and interesting characters in the SW universe since the late 70s. There is a ton of EU and fan fiction about him as well which stoked the flames. I will be shocked if a Boba Fett background movie isn't a complete shit show which panders to the lowest common denominator in order to sell Boba Fett action figures, t-shirts, and underwear.
Sticking in a prequel cameo of a young Han Solo in the prequels would've been dumb and completely unnecessary. Sticking in a young Lando would've been dumb and completely unnecessary. I can see it now:
NERD #1: "HEY! Look! See that 10 year-old kid in a white shirt and dark vest over Jar Jar's shoulder? That's a young Han Solo!"
NERD #2: "OMG! You're right! Wait - who's that over by Qui-Gon Jinn, next to that pod racer?"
NERD #1: <straining> "HA! That's a young Lando Calrissian! You can tell by the cape and his Bespin outfit! He is a pod racer mechanic!"
NERD #2: "I will just die if this is where Han and Lando met!"
NERDS, IN UNISON: "Eeeeeeeeeeeeeeek! Han and Lando are talking!"
This is the kind of lame shit Lucas pulled. Unless they played a pivotal role in those events, giving every character a prequel cameo would be nothing more than pandering to get more toy sales and would be dumb. The prequels were dumb enough on their own and didn't need any additional assistance.