Plenty of Americans watch soccer. You have to remember the 80s and 90s were when soccer took off - they are now in their 30-40s played soccer and their kids play too. World Cup always has high TV viewership, going back to 1994. Even the women’s world cup has high viewership.
The 1994 WC in the US is still the most highly attended WC of all time.
Since now Premier League/La Liga/Bundesliga games are all available live in the US, the popularity of those leagues has skyrocketed here in the US, along with the UEFA Champions League.
The MLS will always be a minor league, compared to the big Euro leagues, but even so, they are focusing on creating a good fan experience.