Why do people use “burner phones” rather Signal or similar solutions?

Why do people use burner phones rather than Signal or similar? I would imagine that if you are high value target the police or intelligence service would simply eavesdrop every phone call passing the base station nearest the location where you dwell. Doing that then it is easy to filter out all phone calls made between phone numbers that have been used more than hundred times or for more than three months (or something like that) in both ends. The amount of remaining phone calls should minuscule, and there you have your criminal, spy or whatever speaking in cleartext on a non-encrypted line.

if you instead of using a dumb phone used something more modern you can on the one hand be exposed through a trojan or a rootkit but is that really a doable attack vector, even for very valuable targets?

If you use something like OpenBSD or one of the hardened Android phones that are on the market and only install Signal on it and then only use it for Signal. Never browse the web, never use it for regular phone calls and texts etc and keep it updated (Maybe regular factory resets too?), can the authorities, even if they have a lot of resources, really manage to sneak in some malware on it? Malware that can’t be caught by another layer of security (E.g., your firewall could be setup to only allows device A to connect to internet to Signal destinations and ports using the Signal protocol – making it difficult to deliver whatever the malware manage to find to the people controlling it.).

What are the best practises for safe phone calls if you are a private citizen that some authorities are very interested in listening to?