If you MITM the connection locally it triples the computational cost for both encryption and handshake operations. Then more websites don't use TLS because it's three times as slow for the user.
It also prevents you from using a good cipher suite when the MITM doesn't support it even though the browser and the server both do, again reducing security or performance or both. And it's very easy to screw this up the other way and have the browser show a good secure connection with strong primitives and forward secrecy while the MITM is actually communicating with the server using export ciphers or RC4.
The existence of a trusted root private key on your machine exposes you to KCI of all servers. And key compromise is not even necessary if they use the same root private key for everyone, which has actually happened.
This is not a comprehensive list of the reasons why that is a bad idea.
Compromising TLS is an infection vector. People regularly download programs from trusted websites and run them. Some apps automatically download updates from the vendor's site via TLS.
AV scanners do not have a 100% detection rate. Letting malware be where a trusted program is expected is how you get infected.
If you MITM the connection locally it triples the computational cost for both encryption and handshake operations. Then more websites don't use TLS because it's three times as slow for the user.
It also prevents you from using a good cipher suite when the MITM doesn't support it even though the browser and the server both do, again reducing security or performance or both. And it's very easy to screw this up the other way and have the browser show a good secure connection with strong primitives and forward secrecy while the MITM is actually communicating with the server using export ciphers or RC4.
The existence of a trusted root private key on your machine exposes you to KCI of all servers. And key compromise is not even necessary if they use the same root private key for everyone, which has actually happened.
This is not a comprehensive list of the reasons why that is a bad idea.