I think answer is ABE. Because Phone Call, Task, & Appointments can be converted to Opportunity or case- Reason being, you capture these records in CRM with relation to an existing record like contact, account, order etc., thats why system does not gives you convert to lead button
Email can be converted to Lead, Opportunity or case- Reason being, email can be received from an unresolved recipient which means email id might not exist in the system; thats why system gives you convert to lead button
Its by design but if you still want it, you can do it by multiple ways like creating a plugin or custom workflow or a real time workflow. Simplest would be creating a real time workflow. You will have to create 3 different workflows each for task, phone call and appointment.