That's because he was being used by Mantis, once Mantis found a better vessel, he dumped Volgin, he wasn't really in charge of himself.
Many characters were misused for sure, but I'd say for having been used "on the run" in an "unplanned" franchise, they could've turned out way worse.
I wonder how the series would've turned out if Kojima had planned the franchise in advance since MGS1.
Btw, does anyone have a screenshot of the timeline shown in MGS4? I read somewhere that supposedly there are two ? marks where PW and MGSV take place in the timeline, and I didn't even remember there was a timeline shown in MGS4