The DOJ wants to break what it considers to be Google's monopoly, and Chrome is a prime target. The problem is that Chrome by itself is worse than worthless, it is a money sink and it only makes sense as a part of a system.
OpenAI is starting to feel the competition. ChatGPT is no longer the only game in town, DeepSeek happened, Google is becoming actually good, Claude is quite popular among coders, and Grok is not a joke anymore. They need something if they don't want to lose out, and buying the most popular browser to make it into a gateway into their service may be an option.