If tenured staff are virtually unsackable, why is the drive to find funding so strong? Tenured staff are virtually impossible to fire, why is the urge to find funding so strong? For post-docs and other untentured scientific staff I can understand, for their job depends on it. For a tenured professor it would rather be the joy and honour of doing important research. Apart from a reduction in joy and honour, are there any consequences if a tenured professor fails to get grants?
In many departments, tenured professors can use bringing in external funds to the department as a means of "buying" their way out of some of their teaching and administrative commitments. Similarly, other departments might use additional committee assignments and teaching loads to "punish" people who don't bring in grants. They may also have less flexibility in selecting teaching assignments.

In other countries, such as Germany, a long-term shortage in funding can lead to the consequence of a chair not being "succeeded" when the holder retires; in that case, the institute (equivalent to a US group) the professor is in charge of is wound down rather than finding a new leader for the group.