I got a working colleague who's just like that. Complete sociopath, doesn't spend one dime on anything, doesn't make bad money but lives in a shitty social flat, always goes showering at work to save money, never goes out, has no car, no dog, no motorcycle, running around in the same shitty clothes all year etc. etc. He doesn't even have a fridge at home. Spends about 3-5 Euros per day on food and that's it.
But dude is mid-twenties and rumors are he has a six-figure sum on his account meanwhile. Don't judge people, you don't know their motifs. Maybe they have huge plans for the future.