I had an idea to assign my keyboard's macro keys to various passwords so I can just hit the key and it will paste in (the passwords are long and random so I won't remember them). This is on a home desktop PC. Are there any reasons this is a bad idea security wise?
This is a bad idea.
As user mentioned in a comment, anyone with physical access can just press the assigned macro key and the password will be revealed.
You also have a high chance of accidentally pressing the macro key, thus typing in your password in places where you didn't mean to.
The macro data also needs to be stored somewhere, and is likely stored in plain text. As such, storing your password in a macro is akin to storing it in a plain text file.
Instead, use a password manager. An offline password manager allows you to store your passwords in an encrypted form, protected by a master password. Many offline password managers also include functionality to write your password for you, just like a macro, except that this is much safer.