What? No, the ransomware people truly do send a decryption tool, or the decryption functionality is built into the ransomware. Do you think they are sending people some AES key and then everyone goes off and builds some python tool to decrypt his data?

This is a fundamental misunderstanding of the ransomware business. The whole reason people pay up is because the hackers don't run and leave you hanging; if you pay they will decrypt your data. Trust and convenience are essential to making this work.