Although there is some variation in rating systems (in chess), most systems put the starting value and the mean of all users somewhere around 1500. Some pools are higher, some are lower, but most have their means somewhere in the 1200-1800 range. As ratings only have relative meaning within their respective pools, scaling or translating all ratings within a pool will not make the rating system any less effective for assessing relative strength. So my question is: what is the historical reason that the means are usually in the 1200-1800 range? Which "natural" parameter choices led to these means? And why not e.g. 100, or around 0, with positive and negative ratings?
For newer rating systems that aim to replace older systems, I suppose the mean is just a consequence of trying to mimic the previous system as well as possible. Say Glicko replacing Elo, or Elo replacing whatever was before that. I've tried some digging, but some historical works are behind paywalls and I am unable to find the historical, initial motivation for parameters choices that led to these averages in the 1200-1800 range.
Any pointers to relevant (freely available) literature explaining this choice would be appreciated!
Edit: As pointed out, not all systems start out at 1500, and averages for e.g. USCF or FIDE ratings may not be exactly 1500. My question is specifically why, historically, someone chose approximately 1500 as the average or starting value (and not any other value), and who was the one that made this deliberate choice (rather than making the average be in the 1000-2000 range to mimic other systems). Maybe the initial choice was just "let's put beginner ratings at 1000" and the 1500ish mean followed from that, that's possible too; I'm just wondering what ultimately led to the 1500ish mean of many rating pools.
(If anyone has access to Elo's original book, maybe they could look up what motivated Elo to use something like 1500 as the baseline.)
Edit 2: While the formulas of the Glicko system may somehow incorporate 1500 as a "special" initial choice, observe that the Elo system is the predecessor and clearly does not need 1500 as the baseline. And surely Elo didn't choose 1500 as baseline only because Glicko would later "need" 1500 as the baseline.
So perhaps to make it more clear that I am not looking for tangential discussions about whether the 1500 value for Glicko is necessary, let me ask the closely related question to my original question: why did Elo use initial parameter choices that led to ~1500 as baseline? (That answer might be "because the predecessor to Elo used 1500 as baseline" so digging further may be necessary, but all discussions about Glicko's subsequent choices are irrelevant to this question about the historical motivation of Elo and its predecessors.)
Edit 3: As pointed out in the comments, sources suggest the Elo system was designed to give similar ratings/rating chances as the Harkness system used previously. So to find the answer to this question one has to answer the question: why did the Harkness system have/lead to 1200-1800ish rating averages?