Your conclusion is far too sweeping but there is a grain of truth to it. Major American rock acts tend to be solo while major British acts are often groups. This has been the trend since the 50’s when most of the great acts that came out of the first wave of rock n’ roll were solo (e.g., Elvis, Chuck Berry, Little Richard, and Jerry Lee Lewis). As to why, perhaps it’s because–deep down–individualism is stronger in America than in Britain.
This is not to say there have not been great American rock bands. A couple I would mention would be The Replacements and REM (who I know some people may regard as being too “soft” to be considered rock). Likewise, there have also been a few notable British solo rock acts like Elvis Costello and Elton John (even though he really hasn’t done anything interesting in 30 years).
Also, you mentioned Lou Reed as being a solo act. However, before going off on his own, Reed was a part of The Velvet Underground which was a highly influential American rock band.