To my best understanding, the BBU also uses ASIC to construct modulated symbols for RU. The necessary data throughput combined with low latency and time-critical operation while keeping low power consumption is simply not achievable in SW. As one mmWave example, the BBU has to keep track of current spatial position of UE and update the beam steering information to track UEs in motion. Most of these decisions have timings expressed at 1e-7 seconds order of magnitude, while SW used can react with 10ms precision. I would be interested in any info that shows SDR being used as part of radio.