What is the meaning of "Manuscript submitted"?
Analogy: I went to the grocery store yesterday.  I was "in line" for 10 minutes, while two customers got checked out.  Why was I not "with cashier" immediately?  ;-)  Capisce?

They also may run some sorting process ahead of time (e.g. which editor gets it).  Or maybe it's a poor paper and they let it sit for that reason (even though the status doesn't say that).  Very hard for us to guess.  Every journal is different, and even different experiences may occur at same journal. 

By the way, some journals do emphasize a more caught up workflow.  At least getting it out to first review ASAP as this is next bottleneck (or desk rejection if not meriting review).  But you will have to learn from experience how different journals in your field behave.

P.s. The answer to your title question is "manuscript submitted" just means "manuscript received".  It's just acknowledgement they got what you sent.