If that was _all_ they did for the past 20 years to get to where they are today, there wouldn't be any problem. The issue is when you leverage this market gain to punish competitors and expand into spaces at below cost (read: negative profits) to wipe out competitors and gain footholds. See examples regarding: Maps (Mapquest), reviews / answer boxes (Yelp), among others.
As a parallel example: Amazon reinvested profits to near 0 margins in their core business for years so that they could legally gain market share without resorting to anticompetitive behavior.* That's why you don't see similar breakup dialog against Amazon: they don't use this market dominance as leverage to own other markets in the same way Google has done. (Now, if they had done this for, say, boosting market share for AWS, then it _would_ be problematic.)
[*] There have been some examples pointed out regarding using third-party seller data in nefarious ways, but as of current evidence and testimony those examples have been so limited that the impact to market share are virtually nil to marginal at best.