You pay $100k a week for a bunch of ivy league/top uni graduates who are very intelligent but lack any real world experience who ultimately only deliver a slide deck and some recommendations so they don't have any real liability, a 21 year old grad can't navigate a lay off round properly and can only provide these 'happy path' recommendations, similar to how a lottery winner won't make good business owner, they have all the money but not the knowledge, the Mckinsey junior has all the knowledge but no experience. Execs hire them because they don't want to be the decision maker only the proxy decision maker and can say 'McKinsey advised this was the only solution to do layoffs/restructure/unpopular thing and they're the ones that know' when in reality all the exec needed was a short stop to back them up.
Obviously this only relates to their management consulting/advisory arms