Sai Points reward good play, not just lots of play. Here's what goes into your score:

• Your average score per game matters most. Score well on each game and you climb fast.
• Playing more games still helps, but the bonus grows slowly. 1000 games gives you only about three times the volume bonus of 10 games, so endless grinding won't carry you to the top.
• Staying consistent pays off. If your scores swing wildly from one game to the next, you lose some SP to a consistency penalty.
• New players start close to a neutral baseline. You need to play a handful of games before your true skill shows up in your SP, so a single lucky game won't put you on top.
• Hard mode counts for 1.5 times as much, so brave players get more SP for the same effort.
• You need at least 3 games before you show up on the leaderboard.
• Ranks are awarded relative to the current leader: whoever sits at #1 is always Master, and the rest of the ranks scale down from there.