Why are recycled aggregates and recycled concrete aggregates not suitable for “High strength concrete”?

Not all aggregates are suitable for producing “High strength concrete” because the strength of aggregates may control the ultimate strength of concrete once they are crushed apart before the failure of cement paste.

Recycled aggregates and recycled concrete aggregates are normally not recommended in producing “High strength concrete”. Owing to their intrinsic effect of reduced compressive strength, it requires increased cement content to counterbalance this effect in normal concrete situation. However, in the case of “High strength concrete”, the very high cement level has already been adopted which offers little scope for further increase in cement content.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.