Everybody is in every week
The virtue of Pick’Em is that nobody is ever eliminated and nobody is ever out of it by much. A survivor pool loses most of its players before it ends, and the people who are out stop paying attention. A Pick’Em pool has the whole field picking in the last week of the season.
It also asks the least of anyone. There is no strategy to learn and no resource to manage — you just pick the games — which makes it the format that works for a group where half the people follow the sport closely and half do not.
What counts as the slate depends on the league
In football the slate is the week: every game is on the card. In basketball a week is roughly fifty games, so the card is a chosen shortlist instead — five by default, weighted toward national broadcasts, and adjustable by the commissioner.
That is the same format behaving sensibly in two different seasons rather than two different products.
Nothing to mark
Scores arrive off the live feed, so nobody types a result into a spreadsheet. Picks grade themselves as games go final, standings move on their own, and the commissioner finds out who won at the same time as everyone else.