You are ignoring how many baseball stadiums are half full, and how many NBA arenas are half full and how many NHL rinks are half full. Most of the MLS stadiums are built just for soccer and aren't that large, but they are full. In Seattle's case they are lucky to be playing in an NFL stadium, but they still bring 45,000 average. Last season, the Mariners had average home attendance of 21,747, their stadium capacity is 47,447. More seats there than the Sounders average attendance. Houston Astros, average home attendance was 20,303 stadium holds 40,963, Houston Dynamo had 19,923 in a stadium with a capacity of 22,039. Portland Timbers had average attendance of 20,674, they play in a stadium that holds up to 22,000 when they expand it.
Soccer stadiums are almost always smaller than their counterparts, and have more attendees.