It isn't about logic, these signatures (even when they were in incoming emails) have been there for as long as I can remember, it is more about history.
When they were there people complained about them because they were enabled by default. Certainly for some time they were disabled by default. I have no idea if this is the case now as it is an extremely long time since I every did a clean reinstall.
This topic started out as you complaining why they weren't in the inbound emails. I simply straightened out the why that really wasn't necessary and the fact of life that the written word (in an email) is just that not a guarantee.
Then you got all upset at the logic of it all, don't shoot the messenger, just disable it if you now don't like the idea of it.
If you now see that the value is not a 100% guarantee to the recipient. If however it is from someone you know (and you know it isn't a spoofed email), checking the email headers, etc. etc. If genuine then it shows you care or the sender cares and the email has been scanned.
We live in much changed times now and 100% guaranties are pretty much gone.