Because one may want to buy tickets for the future, and buy the railcard in the future, thus ensuring no wasted validity. Why add on purchasing time for the rest of use to make up for the minority who can't be bothered to check an expiry date?
Perhaps you should be allowed to purchase a Railcard upto, say, 12 weeks in advance - so long as you will meet the criteria for the Railcard on the date that it becomes valid? And subsequently allow advanced ticket purchases within that same window, for travel withing the validity of the Railcard. Of course, that would also add yet another layer of complexity to the Railcard schemes and potentially result in people trying to use discounted tickets in the 12 week period where they are not valid for travel.
Maybe this is a silly suggestion, but it seems like it should be a relatively simple one to implement: specifically using Trip.com's app (not the website), you can already buy a railcard and specify a date up to about 12 weeks after the purchase for the railcard to begin its period of validity (possibly just a matter of Trip.com not issuing the railcard until that date). Other sites such as TrainSplit's site and the official railcard website lack this option despite Trip.com's development team clearly seeing some value in implementing the feature (and, well, National Express provide the same option for their Coachcards too). I realise that digital railcards are usually issued instantly, but I suspect many still buy theirs early only out of financial convenience at a particular moment or out of a desire to have everything for their future trips prepared in advance, despite it strictly costing them more in wasted validity. Why not simply have more/all railcard vendors provide the option to buy a railcard to become valid only on a future date?
Especially with the Disabled Person's Railcard, and physical plastic railcards, only being available from the official website - and considering it was mentioned earlier that the website already allows ordering a new 16-17 Saver or 16-25 Railcard up to 14 days in advance of a child's 16th birthday, the necessary logistics to allow such advance purchases, including for physical railcards, must already be implemented, the choice for the customer is just missing. As-is, someone moving from a 16-17 Saver to a Disabled Person's Railcard, for example, is caught between a desire to buy their new railcard early and so losing the overlapping validity, or buying it as soon before making a journey using the new railcard as possible, then running the risk of eligibility paperwork processing (which is said to take "up to 5 working days") or delivery taking a little longer than expected; which would be averted entirely with this improvement. Maybe that's a niche scenario, but I'm still sure many using, say, Friends & Family Railcards would find use in the utility too, and a basic implementation could help make future solutions much more feasible or difficult to argue against.