[EricH]

Here are some reasons: Cost. Time. Resources. Strength.

The team I am currently mentoring is a "low-resource" team. No welder. Hand tools only. It's taken us long enough just to get the thing running...imagine if we made a custom frame. Also, it's free, and I personally have NEVER seen one break. Not even bend. (I did tell the team to loctite the bolts for the final build. Hopefully they will, so no robot droppings.) Plus, it's extremely versatile--I can think of three different drivetrains that would just slap in in about 10-15 minutes.