Not all NFL contracts are fully guaranteed. Among nearly 2,000 players under contract in 2022, only 112 had fully-guaranteed deals.
Reasons vary on why a player would have a contract fully guaranteed. Some demand that in negotiations, while others can only sign what they can due to some restrictions.
Rookie contracts for players drafted in the first round see their contracts go fully guaranteed. However, that's part of the rules which also limit how much those rookies can make.
For example, Sauce Gardner was among the NFL odds leaders to be drafted first overall and was selected fourth by the Jets last year out of Cincinnati. Because of limits based on where he was drafted and his position, he signed a four-year, $33.5 million deal. It was all guaranteed.