It's true that at the top they spend a ton of money for slight improvements. There's even a big group that tries to copy them, as if a fifth placer in a local race would've won if only their bike was $5k of carbon fiber.
But that is almost never necessary. When starting out, a $100 used (not stolen) bike will do you well for years. Nerds with $3k bikes and full lycra get passed by folks in shorts and a band t-shirt on old cheap mountain bikes. If a person gets competitive, getting a more appropriate bike (road bike for road riding, for example), it might jump up to a $500 used (not stolen) cost. Folks on 80s steel bikes still beat $3k bike nerds in local competitions.
Plus just like in many other sports, there are all kinds of variations, including levels of competitiveness. Like with soccer, mentioned throughout this thread, most people are playing for fun and with pickup games. They have to buy a ball to get started ($10-$50). Once they start getting competitive or just more invested, now they need socks ($10), shin guards ($80), shorts ($15), and shoes ($100+). They'll need that kit for pickup games that are slightly more competitive. Go up a level (on a team in a league) and now they want better shoes ($200) and new shinguards for their preference ($80) and they have to buy a jersey or shirt and maybe even new socks ($50-100). They have to get a gym bag for carrying their stuff to and from events and practice ($20). If they move up to better teams etc etc they will end up getting a whole new iteration of kit and get into personal optimization and travel expenses, etc etc.
But a person can also just play pickup games or go back to them. Or play futsol for fun. Or juggle in their own yard or street. Most people that ride bikes, even those who want to go fast and occasionally race, are mostly just having fun rides of various kinds that don't take any input outside of routine maintenance and wearing padded clothing for long rides.
Cycling has a bigger opportunity for a person to obsess over an expensive material object for sure, though, no doubt. But most expensive bikes, for most people, are really about a consumption hobby more than the activity. Many of the expensive ones you'll see in the street are in no way race-optimized. They are big hunks of steel or titanium with parts milled in the imperial core in batches of 10-20, purchased for aesthetics. They could have spent $300 on an older bike with better steel and fixed it up, but they are instead in a consumerist subculture where they got a brand new, lower quality frame for $800 on its own because it says "soma" on the side. A better new frame from China costs $200-300. And all of these bikes... they just look like normal bikes. Maybe one is orange and the other is blue. But a person with extra cash looking for a consumerist hobby will find options like thst $800 ho-hum frame and obsess over it.
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Gross. I'm sorry you have to negotiate this, I am sure it is difficult to square any caring and familial and friendly feelings towards your brother with him joining the latine harassment squad.