It is how things work legally. What you are confusing is the distinction between individual bits of information and a database: if a salesperson leaves and calls their old client, nobody reasonably expects them to forget about that relationship or be unable to look up a phone number.
If there’s an entire lead database, that might be a different story if it includes non-public data and the company can show that it’s treated as a valuable asset (limited access, confidentiality agreements, etc. ). If it’s something you could recreate with a few Google searches, you’ll have a hard time convincing a judge that there’s substantial value in its secrecy.