Quick answer: IPv4 only allows about 4.3 billion unique addresses, and that supply was effectively used up years ago as the number of connected devices grew far beyond what its designers anticipated.
How the internet copes
Techniques like NAT, CGNAT and IPv6 all help stretch or replace the limited IPv4 supply. This is exactly why your own connection may share a public address with other subscribers.
IPv6 is the long-term fix
IPv6’s much larger address space is designed to give every device its own globally unique address again, without needing extra translation layers.
See What Is CGNAT? and What Is NAT? for how this plays out on an ordinary home connection, and check which version your connection is using with IP Lookup.
Want to see this in practice? Check your own connection's IP, location and ISP.
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