If linux tried to add pledge you'd have people coming out of the woodwork claiming that linux was breaking UNIX and requiring "linux specific" patches to every project.
Second, and much different from what SystemD just did, pledge does not affect any program that doesn't use pledge. From an external caller point of view, each changed user land program should act in the exact way that it did before the change.
A programmer can safely ignore pledge and nothing has changed for them. A user can safely ignore pledge as it shouldn't change their usage.
If Linux (I would guess more appropriately the maintainers of the GNU user land) tried this under those conditions, it would still be an internalized change that shouldn't affect anyone outside the project except in the positive as an exploit mitigator.
The bottom line is: OpenBSD is not requiring changes in other developer's programs or in user usage to continue to run in the same way they have run in the past.
Now SELinux isn't perfect by any meaning, but the key difference is that applications can be SELinux aware if they choose, but you can still have full protection without the application knowing that it exists.
I would much rather try to improve SELinux than try to make pledge work for precisely this reason.
Developer knows better what program should and should not do and is much more capable to set this kind of rules with pledge.
And that concept in turn means that a general can put a trusted system into hostile territory and trust it to not leak secrets.
And NSA still care for it. Around the time of the last kdbus dustup, someone from NSA voice a worry about how kdbus would compromise certain parts of SELinux.