Back in season 3, when Oliver wanted to push Diggle off the team because of baby Sara, Dig told Oliver that it was his life and, despite Oliver’s objections, he was choosing to continue being a vigilante. Having a kid didn’t change that for Dig. It gave him one more thing to fight for. Since he’s still on the team three years later and even ran the team when Oliver and Felicity ran off together, I doubt he’s changed his mind about that. This has become Dig’s full time job and that was his choice (as we found out courtesy of Curtis asking the real questions).
Oliver is choosing different, right now at least, and his circumstances are a little more complicated then Dig’s were. It’s not just having a kid, and especially and older kid who can voice his fears about becoming an orphan, but also he’s got the FBI on his back and idk but his relationship with Felicity also might factor into his decision in the next ep.
I also think Oliver’s got some blinders on right now, with being a dad and the mayor and things. Diggle hasn’t said anything to him about his problems and Oliver hasn’t been paying 100% attention. It’s created a perfect storm of problems that’s gonna bite them in the ass at the worst possible time and I’m really excited about it!