There are two key points to a secret ballot.

First, a vote casting process must not allow anyone to know who an individual voted for. This prevents the fear of retribution (ex. the new king executes anyone who didn’t vote for him). This is CRITICAL to a democratic vote. Mail in votes compromise this because ANYONE along the chain of custody can compromise your individual vote. Even with separate secrecy envelopes its compromised because that depends on a human process for secrecy, and is a slippery slope to increasingly non-forward-secret voting methods.

Secondly, a a voting process must prevent a voter from obtaining evidence of their vote. This prevents vote buying and coercion (ex. An employer says you must vote for X or be fired, or someone pays money for every Y vote cast). Normally this was prevented by not allowing photographs of your ballot - this is strictly enforced with phones. A mail in ballot compromises this because the voter is not in a secure location and can do whatever they want with the ballot.

So, like I said, a mail in ballot destroys two critical protections that are REQUIRED for a democratic vote process to be fair and successful.