Why does a stem typically contain more sclerenchyma and collenchyma than does a leaf?

Answer 1

Yes mostly, but not always. Stems are more likely to require structural strength provided by sclerenchyma and collenchyma than leaves.

Answer 2

They typically contain more collenchyma which is thicker in the corners of their cells and sclerenchyma which are reinforced with lignin because they offer more structural support which is needed in stems to protect against wind or other factors that would cause the plant to move.