How so? Bad actors buying existing extensions with large user bases then publishing a new version which does bad stuff is a pretty common pattern. It certainy seems like a reasonable concern for a corp IT department.
When there are unpatched browser vulnerabilities, attackers will use ad networks to inject attack code into reputable-but-ad-laden websites. And even when there aren't unpatched vulnerabilities out there, many ad networks will happily accept scam ads, ads that trick people into downloading malware, fake download buttons and suchlike.
But if they all use Chrome, wouldn't those be really weak ad blockers?
I'm extremely intolerant to ads, so I would leave Chrome if ad blocking stopped working.
If everyone had the same attitude this would probably already be the case.
100% of security experts I know find ads annoying and know ad blockers reduce how many they see.
I figure they had a switch they could toggle and they thought no further about the tradeoffs. Because their primary concern is their own liability, not what's best for the org their contract is with.