Every dollar paid in fund expenses is a dollar that never compounds for you. Over a single year the difference between a 0.03% index fund and a 0.63% actively managed fund looks trivial - six dollars on a thousand. Over thirty years, on a growing balance, it is the difference between retiring on schedule and working two extra years.
Consider two savers who each contribute $800 a month for thirty years and earn 7% gross. The low-cost investor ends with roughly $975,000. The high-cost investor ends near $860,000. Neither picked better stocks. The gap is entirely fee drag, and it grew every year because the fees were charged on a larger and larger balance.
This is why our default building blocks are broad, cheap and boring. Diversification removes the risk you are not paid to take. Low costs remove the drag you cannot control the market to overcome. What remains - your savings rate and your time horizon - is the part you actually control.