It's harder to earn money if you only let your sims have jobs. I think it's pretty much impossible to play on normal lifespan and have a couple earn 750,000 + a big house from just jobs. It's the activities like gardening, painting, etc. that make sims filthy rich. If you cut back on those things, you'll have more of a challenge.
ETA: Even if two sims began YA at the top of their careers and worked every day for 5,000 simoleons a day, which no sim does, they still wouldn't earn 500,000 simoleons from careers.
5,000/day × 48 (YA + Adult length) × 2 = 480,000.
