I think league play is more key to amateur tennis than to golf b/c when it comes down to it...you can play the game of golf and follow all rules with just yourself as the primary opponent. You can't play the game of tennis solo, nor can you play without having a winner and loser. You can rally, you can run drills, but to truly play the sport of tennis, you have to keep score and someone has to win, someone has to lose. In golf, you can play the sport and keep score, but there doesn't necessarily need to be a winner/loser for the game to be played.
This makes tennis more competitive by nature and people who play it tend to enjoy that winning/losing element of the sport. Meanwhile, golf has a lot more casual players who are trying to beat their own scores, not the scores of their playing partners. The handicap system was developed in part to make competition among amateur players more even, but most amateur golfers don't even carry an official handicap from GHIN or other index, they just have an estimated one if that. that said, you can still play 18 holes, count your strokes, record your score and you have played the sport of golf without "winning" or "losing" to anybody.
I love golf and I love tennis and I play both regularly. But they fulfill different things. I play tennis for exercise/competition/quick social thing that takes less than 2 hours and I play golf for fun/4.5 hours of social time with a couple friends/some peace and quiet.