Well I understand a little bit more your point. But you are not the "average Joe" as you say and very few people have your skills to be able to spot weak libraries and functions related to entropy generation in software wallets. For the common user it's basically a black box you need to blindly trust. So personally I prefer sticking to known pretty reliable physical sources, even if they are not radioactive like Balthazar's stones 
To be fair, when you're converting your entropy to a seed or an address, you're also blindly trusting the script as well. Some of them are mostly unaudited and have certain inadvertent bugs that could reduce entropy. Iancoleman had this with their manual entropy generation. Understanding entropy isn't just about trying to look at a few lines of codes.Most users don't look at what they run and wouldn't understand the code behind it either, even if it is as simple as trying to generate a seed from a given set of entropy. Most well known wallets are very well audited nowadays and has far more stringent checks in this respect.
At least Ian Coleman and Bitaddress conceptors seem to agree with me.
JavaScript used to have fairly weak CSPRNG and randomness is not guaranteed as the recommendation is just for browsers to implement the correct entropy sources. JS is generally just worse off in terms of cryptographic security.Sure, you can generate your own entropy. Whether you're able to do it securely and without any loss of security is debatable.
There is a reason why major wallets don't allow their users to randomly specify their own entropy.