No that isn't why. The food index increased 0.4% for February, and also the main CPI number published by the BLS is not Core CPI, so the number in the headline is not what you implied it is nor is the .4% increase in food out of line with the aggregate index.

Because anti-trust jurisprudence in the US has been incredibly poor for the past 30 years. The courts have in general been much more willing to allow industry concentration and will put down anti-trust suits pretty often. It's part of why pretty much every industry has seen a trend towards concentration in the past 30-40 years without much pushback from the federal government and why the last major successful anti-trust action was the Bell breakup in the 80s.

edit: and AT&T only had a 45% market share before the breakup.

Anti-trust action doesn't usually require one to have an actual monopoly in a market. Being sufficiently large and concentrated such that one can influence the market in an anti-competitive and bad-for-consumers manner is all it takes. And 60% is certainly enough to trigger that.