A number of reasons.
1) Some people get nervous and "take all their money out" and suffer deep consequences. We saw this back in 2008 where people panic. especially older people, when they look at their account and they've lost tens of thousands or maybe even a few hundred thousands of dollars in their account.
2) Some retirees have emergencies and have to take money out and taking large sums of money on a downturn is brutal for someone's personal financial situation. This also applies to people that need to access money in 529 plans.
3) There is no debate with investment professionals or even moderately educated investment novices about not taking out all your money as you get close to retirement. They all advise reducing risk, moving their funds to more stable funds within their portfolios, but also keeping a portion of their funds in growth funds so they can take advantage of growth opportunities. Given retirement can last 20-30 or more years, having a decent portion of a portfolio exposed to continued growth is desired and necessary for most retired people.
4) I have not asked my father in law recently, but I know just a few years ago, my father in law told me he had as much money in his retirement portfolio at the time as he did when he retired almost 15 years before. That was after RMDs and other withdrawals for extensive travel and everyday living. That was due to growth and leaving a meaningful portion of his funds in the market within growth funds.
5) There are fee only financial professionals that don't make any money on your funds or what funds you choose, or if you get in or get out.
6) I don't have a financial professional. I do my own planning and have since i started investing in the market when I was 23 years old. I've also helped my son, a Junior at USC, start his own investing portfolio.