Top 10-2017 A8-Insecure Deserialization , why products does not implement encryption
Why are products not encrypting the serialized data as discussed in [#30723]? 


Because encryption by itself does not reliably prevent alteration of the data, which is the vulnerability here. As Maarten Bodewes comments in that very Q.

To prevent tampering in general you need -- as OWASP says -- an integrity check, like a MAC or signature. Not encryption. (Although if you have authenticated encryption, which does both encryption and MAC as a combined operation, and in recent years has become more popular and widely available, you might as well use it.)

Now, why don't/didn't people use a MAC or signature (or AE) on data which if altered can be dangerous? That's a good question. To which I don't have a good answer.