And that's not how the Parisian RER works - they have very few stops in the city of Paris itself, to prioritise long distance travel and make it faster (somewhat as a reaction to how unsuited the Paris metro, which has a ridiculously dense stop network, is for long distances).
Amersham is a 13th century market town. It's not tiny, but it's certainly not a city.
Now, there are mainline services running on the same physical rails which do move passengers between London and other cities, but those don't turn into a stopping underground service, the trains leave those shared rails inside London and run to a terminus station, Marylebone. Marylebone is an Underground station, but only in the same sense as say King's Cross or Waterloo - the Underground is separate though located in the same warren of buildings and tunnels.
The Elizabeth Line is closer to the model explained here, running a fast long distance service, a metro service and then back to fast long distance. Reading isn't a city but it's more significant than Amersham and those are core London services not a terminus with the option to change.
If you wanted to go from Reading to somewhere in central London, it would be faster to take a non-stop express which terminates at the edge of central London and then change onto the Elizabeth line than to take it all the way. The service to Reading is more about providing a local service between Reading and its surrounding small towns.
[1] except Iver
It only has 5 stops with generous distance between them in London/Zone 1, which is equivalent to Parisian RERs. Not really what I would call metro service.
It's interesting because both are true: outside of central london is a metro service, in central london is a fast service. The parent op explains it pretty well in the last paragraph:
> running a fast long distance service, a metro service and then back to fast long distance.
I think it's interesting how the most recent line (n° 14) has way less stops, instead focusing on covering more distance
Funnily this was so successful this led to overcrowding on the 14, hence the extension of the line E to take some of the traffic from the 1, A and 14; and the 14 itself was extended to connect one of the airports and a bunch of suburbs under heavy redevelopment.