It’s a combination of physical limitations and everyone's behavior. Most people download much more than they upload, making asymmetric speeds practical. Standard cable internet uses coaxial copper lines designed for one-way cable TV, requiring bandwidth allocation favoring downloads.
DOCSIS standards improve speeds but cost ISPs money to upgrade. Fiber internet avoids these limitations, offering equal upload and download speeds. Content delivery networks also help distribute download data efficiently.
Source: Engadget · Summarized by HeadlinesBriefing