Probably for the same reason that a lot of the business world is actually run by 70+ year old programming: standard processes were designed around the old technology and all the people who originally designed it are gone so no one really knows how it all works together and are afraid to introduce any new changes for fear of breaking things.
Fortunately, the likes of Garmin/ForeFlight/FltPlan all offer “plain text” translations of the NOTAMs, and more recently, even graphical depictions of them. At least the effort to eliminate the useless NOTAMs will help. More could (and should) be done, but it’s a start.