Given that Stansted Airport is Britain's second busiest airport*, and that it only operates at around 60% capacity, it would be expected that it would have a substantial ticket checking system - I don't even think it has smartcard readers! I would expect this to be improved with Project Oval (assuming it comes out this far).
As someone who lives in the area, I assume that part of the problem with the ticket system (and the overall service in general) is that it is in an extremely rural area for a London airport (it's equidistant between London and Cambridge, the largest nearby settlement is Bishop's Stortford with a population of ~40k), and it is London's only main station not on a "traditional" quad-track main line (2 fast, 2 slow), and as such it (shockingly) lacks overall facilities.
I've only ever found ticket checking onboard SX services to be during the morning peak into London.
*According to last year's data. It has had a very good post-covid recovery, and I know pre-covid Gatwick (and possibly East Midlands) is usually higher