Beetroot is a 'root' vegetable you need to leave the plants in the ground long enough to grow the enlarged root. You also need to water regularly if it is dry.

I'm just wondering from your question whether you might have bought 'beet', meaning leaf beet, rather than beetroot? The seed look identical but leaf beet or swiss chard never produces a 'root'.
Beetroot normally take a couple of months to produce a reasonably sized root and need adequate spacing if you want monsters.