Lack of career growth. Point blank, it is REALLY hard to get promoted and few do before leaving. Pay raises are few and far between despite taking on many responsibility promotions.
Internal mobility is amazing but it puts you back a year in terms of career growth. Cycles for job openings are always before the review cycle so people decide between following their long term career interests or long term career growth / pay. I've moved internally once and this has put me on the same pay as post-grad L1s, despite managing our most important clients and excelling. People are not accurately rewarded for their work.