Why are the possibilities of unicellular organisms in the development of new habitats and food sources limited?

Single-celled organisms consist of only one cell. All life processes proceed accordingly in this cell. Because of this, their adaptability is limited. Single-celled organisms have certain adaptations to a specific habitat, which have developed over many hundreds of thousands of years during evolution. For example – some organisms can live only in fresh water, and some – only in salt water. But despite the weak ability to adapt, they are very tenacious, since many unicellular organisms live on our planet for millions of years.