On the ISP end sometimes non symmetrical equipment is used, especially on copper coaxial which are used much like “wired wifi” in that data is transported by encoding it into frequency bands. Each frequency band can only be used up OR down per cable, so ISPs tend to dedicate more frequency bands to the downlink than to the uplink.

And as others mentioned, the commonly used TCP protocol will slowly ramp up bandwidth by having the server send a burst of packets, the client acknowledges, then the server sends more packets faster and the client acknowledges again, and once the client and server starts noticing packet losses it backs down and resend the lost packets a bit slower, until the connection bandwidth is stable. If you fail to send acknowledgements the server will back down on the connection speed even if you’re able to receive at full speed