ROS messages generally sacrifice optimality for generality. I guess no message defined with byte-based intensity would make it into a standard. Using a float is the way intensities are represented in other standard messages like LaserScan. This is to be “future-proof” and have one definition that works for both integer-based intensities and float-based ones.
So now you’re two floats against five =) Not sure what is the bitrate of this sensor, but maybe even the suboptimal representation would be good enough for normal use.
You can try to jump into this older discussion to ping the people who were discussing the radar_msgs standardization: Radar Messages Standard .