- Video advertising is not happy with only impressions and clicks metrics. In general, advertiser will want to know if their video played while in view from a human (or at least in view on a screen), and for how long it has played (or at the very least a rough estimate, like say how much midpoints).

- The concept of "impression" itself is often not very well defined, but counting it at "the server served a request with the video payload" is really a too optimistic view of things which leaves big holes exploitable by fraudsters. Having client side javascript playing at least requires additional software running, aka additional costs (if minimal) for fraudster.

- You can't really "just" monitor the user on the campaign landing page, since it's different sites involved, it involves different cookies, and actually reconciliating them is doable, but it'd require some work that the advertiser may not be willing or able to do.