While I don't disagree with the explanations that have been provided thus far, the big remaining question marks in my eyes in regards to the absence of Space Marines in the High Lords of Terra are these:
1. Let's say there isn't a centralized chain of command that would indicate who the "ranking" Chapter Master is. It really doesn't matter, does it? It's the High Lords of Terra who promote individuals to the rank necessary to join their Senate and then in turn confirm said appointment. E.g., Lord Commander Militant of the Imperial Guard; Lord Commander of the Segmentum Solar.
2. Also, it doesn't matter if the High Lords of Terra aren't able to track all the Chapters out there. They definitely
can't track all of the Imperial Guard Regiments out there, and I'd be willing to bet they can't track all the Lords General Militant from whom they promote their Warmasters (who are the only candidates for Lord Commander Militant of the Imperial Guard).
Besides, I'm confident that enough nepotism/good-old boy networking occurs within the higher ranks of the Imperial Guard (in regards to promoting people from "more storied" Regiments, or with familial connections)--and that the High Lords are aware of this--for the High Lords of Terra to really worry whether they're giving all the Chapter Masters a fair shake.
Like those other two posts above, they'd likely focus on those Chapter Masters that are both famous for their actions and "acceptable" in terms of sanity, approachability, etc.
E.g., your Marneus Calgars, your Tu'Shans, etc.
Besides, serving on the High Lords of Terra isn't equivalent to serving THE High Lords of Terra. Heck, the Fabricator-General represents a faction that is all but independent in name. I would imagine that, even if the majority of the Chapters thought it to be a pointless formality (which it really wouldn't be), the cannier Chapter Masters out there would see the benefits from such involvement.
As to the mythology surrounding the Astartes (e.g., the Emperor's Angels serving on a Senate)... no offense, but I don't see this as a reason for them not to want in. The Captain-General of the Custodes technically occupies a niche with even more mythology involved ("I am God's bodyguard"), and he has no problem with a Senatorial term.
Just my two cents' worth!