My partner works for UberX while finishing up his grad school degree. While I estimate that the true take-home pay of UberX is only 40% of the quote fare (considering the 20% commission for Uber plus your self-employment tax, federal/state tax, and vehicle maintenance/wear&tear/gas/etc.), it's a decent gig (esp. as a side gig) -- IF you already have a car. I would think that the upfront cost of buying a vehicle that meets Uber's standards (I don't think they even consider allowing anything older than a 2006 to drive for them), registration, etc. would end up being a huge investment. Plus, I'm not actually sure what you mean by "reinvest into more vehicles/drivers". You work for Uber as an independent contractor -- you don't sign up other drivers or multiple vehicles.
The long and short of it is, if you have a decent car and some hours to spare, sure, drive for UberX but be sure to take all of the taxes and wear and tear into account. But don't go out and buy a car just to drive for Uber.