To highlight that I'm not asking for miracles. These games are 5 to 10 years old.
Certainly, but keep in mind it isn't as if there is a big long lineup of games in the timeframe you're asking for lining up at GOG's headquarters kicking the door down, with GOG on the other side saying "Go away, we have enough games already but thanks anyway!" :)
Seriously, GOG contacts all of the game companies out there at every gaming conference and every other possible opportunity they can to try to find as many games as possible that would be big wins to have here, including both new and old games. If there are any games available in the timeframe you stated that are remotely available to them they'd either have them here already for sale or they'd have tried to get them unsuccessfully by now. It takes time to establish connections and garner interest and then to wade through legal rights issues and negotiate terms and conditions of sale etc. If a game isn't here it's not for lack of trying. :) GOG is doing their job to make it happen for DRM-free gaming, and moreso than any other company out there for sure. :)
If games are missing from here that people want, the best way to make it happen is to both vote for the game on the GOG wishlist (or add it to the wishlist if it is not already there - search before adding new games), and to contact the publishers/developers and politely let them know you'd like their game(s) to be sold on GOG.com. In some cases it may involve trying to track down the legal rights owners for a game manually as well, and some GOG gamers have done that for some games too.
In almost all cases where there is a highly desireable new or old game badly wanted here, it is not because GOG has rejected it or because GOG hasn't tried to get it - it is because the publisher/developer/rights owner has either not responded or simply doesn't want to do business here, or they can't find who owns the rights to everything, or the rights for everything is divided between multiple companies/individuals in a complex way or can't be sorted out, or they're unable to come to terms that both GOG and the rights owners agree to. And if we're being really honest it is because some of these companies not only fully embrace DRM, but they do not want to sell their games without it. For the case where the game is already DRM-free somewhere else, it could be that the game is tightly integrated with Steam and they couldn't be bothered to make as special version of the game for GOG, or any number of dozens of other reasons. GOG can't really pull a rabbit out of their hat if a publisher isn't interested in selling their games here, or doing all the work necessary to make it happen, however they do occasionally manage to make some magic happen too.
The key is to realize that GOG can initiate contact and begin the communication process, but they can't force it to happen, the rights owner(s) have the full control over where their games are sold, and if they're not sold here it is the decision of the rights owner(s). So 1000 of us can post "yeah, me too!" here because probably most of us feel the same way, but it doesn't really make any difference to do so because making noise in the forums doesn't really make games show up here. :) There's no magic incantation to make it happen, but the best we have is GOG wishlist + contacting publishers and tracking down rights owners ourselves if need be and providing the info to GOG.
Other than that it's a waiting game. :)