Available funding is constrained complicated ways, and the whole incentive system is a mess.
Departments/PI's can't hire for the positions they would choose, so they hire postdocs for very short terms (often 2 years or less) because they can. Candidates can't get the positions they want (e.g. TT positions) so they take post-docs until they age out or find/settle on something. Everyone understands that there may not be a classic TT position for everyone interested, but the system has so far mostly failed to come up with any sort of viable alternative for long term employment. Not that TT is what it used to be, either.
As a result, continuity in labs is very difficult to maintain as the senior people rotate out too often. Couple this with the growing tendency for PI's to spend more time fund raising than doing research and you have a lot of lost and wasted effort.
Available funding is constrained complicated ways, and the whole incentive system is a mess.
Departments/PI's can't hire for the positions they would choose, so they hire postdocs for very short terms (often 2 years or less) because they can. Candidates can't get the positions they want (e.g. TT positions) so they take post-docs until they age out or find/settle on something. Everyone understands that there may not be a classic TT position for everyone interested, but the system has so far mostly failed to come up with any sort of viable alternative for long term employment. Not that TT is what it used to be, either.
As a result, continuity in labs is very difficult to maintain as the senior people rotate out too often. Couple this with the growing tendency for PI's to spend more time fund raising than doing research and you have a lot of lost and wasted effort.