Why are Sphinx forward messages not used in Lightning's onion packets?
Questions like these are always leading to an opinionated response and are a little bit tough to answer. I will give it a shot though.

First of all I think youelf already answered your question to an important degree. Forwarding messages via the SPHINX protocol yield the possibility of message length attacks.

I think Another reason was the goal to be simple in the beginning. I know there is a lot of discussion to extend onion routing to HORNET which would create a connection oriented encrypted communication layer on top of the lightning network and routing protocol. This obviously yield some advantages but also complexity and possibilities to introduce exploits.
Let us remember that sphinx mix format was created to send and receive messages / emails and we "abuse" it for lightning. So I guess we liked at the core feature we needed. 

I think the main reason for using the EOB field in spontaneous payments is that it was easy to implement this in the current system without too many changes. But to be sure roasbeef might give the exact reason.