Why is symmetric lightweight crypto only about block ciphers and not about stream ciphers? Why is symmetric lightweight cryptography only about block ciphers and not about stream ciphers?
AFAIK stream cipher implementations cost at least three times less than block cipher ones?
Why is symmetric lightweight cryptography only about block ciphers and not
  about stream ciphers?


Why that assumption? There is a lot of work concerning stream ciphers for lightweight cryptography (LWC). You can find a relevant list here.

Plus, last week the ChaCha20-Poly1305 cipher suites (which are considered as lightweight) have been standardized in this RFC.