Why does a virus spend a long time in the lysogenic cycle could be more dangerous to humans?

Cuz they are hidden and have more opportunity because since they hide the DNA/RNA inside the cell's dna/rna, the cell with do mitosis without noticing, and the DNA will spread faster than the lytic cycle.

When a virus infects a host cell, it uses the cell's machinery to make copies of its own genetic material in a process called replication. finally, during the process of self-assembly, that protein shell and the copies of the virus's genetic material are assembled into new viruses.