Managing a portfolio in the years leading up to retirement doesn't require a lot of magic. If you save enough of your salary and invest those savings in even a semi-sane investment mix, you'll have a good shot at amassing enough for a comfortable retirement.
Shepherding a portfolio through retirement, by contrast, is inherently more complicated. You have to make sure that you're not spending more than is prudent and that your investment portfolio appropriately balances safety and growth potential. Ideally, you'd also stay attuned to pulling your withdrawals from the right account types to help keep your taxes down. And those are just the headline jobs: You also have to think about required minimum distributions, estate planning, your Social Security strategy, and how you'll shoulder long-term care costs if they arise. That's a lot.