Why are so many tail-strike incidents not verified until later inspection?
On one hand tail strikes are mitigated long before the need for a sensor. Aircraft makers generally address it from a design standpoint before the aircraft ever flys. If that is not enough control augmentation or tighter procedures are developed to prevent it from an operational level. 

Ultimately such sensors do exist and do warn the crew of a tail strike they may just not yet be certified or installed on every airframe. For example it came in handy in this incident: 


  After the passengers had disembarked, the captain conducted an
  inspection of the tail skid of VZR and noticed some paint damage and
  scrape marks, however the cartridge containing the sensor for a
  tailstrike was still intact. This indicated that the tailskid had only
  just contacted the runway during the take-off.