It's not by definition, there's a bit of math behind it! :)
The intuition is that the hardest problems in P have linear dependency chains. If you could have a good parallel algorithm for a P complete problem, you could take a problem with a dependency chain and solve parts of it in parallel without waiting for the results those parts are depending on.
Cerium is correct, we don't know if P is efficiently parallelizable.
Is there a formal proof of what you're talking about that we can read?
Are you perhaps confusing P with P complete?
https://www.researchgate.net/profile/Walter-Ruzzo/publicatio...
How does that matter? P-Complete is a subset of P.
What am I looking for in this 300 page document? Proof by intimidation, eh?
"The class P, typically taken to consist of all the "tractable" problems for a sequential computer, contains the class NC, which consists of those problems which can be efficiently solved on a parallel computer. This is because parallel computers can be simulated on a sequential machine. It is not known whether NC = P. In other words, it is not known whether there are any tractable problems that are inherently sequential. " From: https://en.m.wikipedia.org/wiki/P-complete