Why are guide RNA libraries supplied on lentivirus vectors but not on regular plasmids?

Using lentivirus vectors ensures that the researcher can control the presence of only a single sgRNA per cell. Plasmid delivery would result in dozens of different guides per cell which would make it impossible to determine which knockout (or combination) causes the observed phenotype.
To determine which gene has been knocked out in any given cell following a phenotypic screen, you need to determine which sgRNA was delivered to that cell. Therefore, the coding sequence for expressed guide RNA needs to be present after the screen, i.e., stably integrated. Lentivirus delivery under a controlled MOI results in a stable single-copy integration.

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