Let's say you have three projects, A, B, and C. Assume each one takes 3 days.

You can work on them serially:

A A A B B B C C C

Or multitask them:

A B C A B C A B C

If you take a look at when the projects complete, you'll see that, in the serial approach:

A finishes on day 3. B finishes on day 6. C finishes on day 9.

In the multitasking approach:

A finishes on day 7. B finishes on day 8. C finishes on day 9.

This results in lower system throughput.

There may be reasons for multitasking, but increasing system throughput isn't one of them.