As of 20 years ago, UMG held something like 3,000,000 "assets" (and Sony somewhere in the neighborhood of 2,000,000). If even only half of UMGs assets were tapes that you wanted to digitize, and each tape only took you 30 minutes to successfully transfer, you're still looking at 85 man years of labor to complete the task. I think Warner tried to digitize their entire library of mixed masters some years back but paused the work amongst budget cuts, and those only represent a fraction of their overall holdings.
With this kind of thing you're also running into questions of "when is good enough good enough?" What do you digitize to, 192/24 PCM? Single, double or quadruple-rate DSD? What happens if digital technology improves audibly 10 or 20 years from now, or someone invents a Jules Verne-ian breakthrough technology for tape playback that can digitally scan or read analog tape with a laser akin to the ELP laser turntable? Surely it would obsolete everything you've done before, and you'd have to start again - I don't think labels want to get into a 'painting the Golden Gate Bridge' situation where it takes so long to do something that you start over again every time you finish, either.
Labels are in the business of spending money on things they can make money off selling - it would be nice if they did digitize everything, but there aren't enough hours in a lifetime for them to complete it, and not enough benevolent shareholders that would support the idea.