Nodule bacteria that assimilate nitrogen live on the roots of leguminous plants, but they simply cannot

Nodule bacteria that assimilate nitrogen live on the roots of leguminous plants, but they simply cannot live in the soil. Why do you think?

Such bacteria can live not only on the roots of legumes. They can enter into symbiosis with other plants. Symbiosis is a beneficial relationship for living organisms. The bacteria help the plant absorb nitrogen into the plant. In turn, the plant supplies the bacteria with nutrients. Thanks to these nutrients, bacteria are able to live on the roots. There are no nutrients in the soil. Therefore, these bacteria are simply not found in the soil. The same goes for many other symbionts.