The capitalists own the means of production. That produces income beyond what they need. The surplus money allows them to buy more productive assets. These assets only need to be sold if the owner has a LACK of money (i.e. they hit a rough patch). Hence it continues to accumulate among fewer and fewer rich people who pass it to their next generation. They are in competition WITH EACH OTHER to accumulate assets.

So what compels them to work hard (not productively, but still hard) to ammass ever more money? Surely it’s easier to sit in a pool all day and collect tithes rather than strategizing about the new merger, the new apartment block to buy, the next politician to bribe ecc.

LOL. They don't work at all. They employ someone to do the work whether it is the strategizing or the bribing.