The musicians are excited and eager and there's nothing to remember or get right. Nothing can go wrong. Everybody knows what a song is. It's no great mystery. The freedom of jazz in its heyday was based on the strictness of the blues structure.
Rules provide freedom.
After a verse comes a chorus. Everybody knows what a chorus has to accomplish. It can, therefore, be invented, on the spot, with confidence. You got 3 things you got a song. In other words, on my favorite record of 1997, the year barely exists.