Cubicle 7 actually sells both physical and PDF versions of their product, and they sell both directly to the public. Wizards does neither - they don't sell PDFs and they don't sell directly to the public. So it's apples and oranges.
If Wizards ever decides to sell the current version of D&D as PDFs directly to the public then I suspect they'd figure out how to do the same as other companies. But since, unlike other companies, they actually don't have to sell PDFs at all to meet the bottom line I suspect it's a very low priority.
And then figuring out how to give people D&D Beyond access for the physical books they buy is a different question - it takes more labor to put together the DDB stuff for a book than it does to produce a PDF for sale (given that PDFs are an intermediate stage in the printing process while DDB programming/data entry is completely divorced from that process). So paying for that labor needs to be factored in somewhere. There are two obvious ways they could do it - raise the prices on books for everyone to provide "free" access to DDB to everyone - even those who don't use it, or have people who use DDB pay for that labor. They've chosen the latter, and I hope they don't ever switch to the former because I just don't see the value in DDB for 5e (digital tools were a real time saver in 4e, but for 5e I just don't need them).