We looked for Gravitational lensing only after Einstein proposed his theory and wanted validation. No one wanted to actually put the effort to do the measurement during an eclipse and iirc Einstein had to lobby for it greatly and only a single lone wolf guy took him up on it. And if I’m not wrong the initial observations weren’t even that solid and were likely artifacts that just in retrospect were interpreted in the right side of truth.

All other observations likely would have taken decades to stumble upon as well. I see no argument still why any other scientist would have made the relativity leap (which to be clear isn’t directly a requirement for deriving E=mC^2).