Do jetliners carry radar that can detect other aircraft?
Radar that can detect, resolve and track small targets as opposed to large areas of water (raindrops) has to be a lot more powerful and sophisticated, especially in ye olden tymes of the 50s and 60s when weather radar was becoming a thing and the search for a practical collision avoidance system was underway.
When the effort that eventually became TCAS got started in the late 50s following several notable midairs, on-board target detection was one of the early proposals but was rejected by the industry as too expensive and time consuming to develop (where to put the more powerful system with its larger antenna for starters).
Once TCAS was perfected, made possible by the development of Mode C altitude reporting by transponders, you got 95% of the benefit (because only non-Mode C-transponder light planes were still a risk - a very minor factor overall once Mode C was pushed out to GA aircraft) without a crazy expensive addition of on-board target detection and tracking radar PLUS a weather radar.