Why Some 21st Century US Rockets Still Use Soviet Era Engines

The RD-180 is an exceptionally good engine, and a big part of that is down to it having a closed combustion cycle, compared to US designed engines which use an open cycle. Why did the US never develop a closed cycle Kerolox engine, and what problems did the soviet engineers solve decades ago.

Видео Why Some 21st Century US Rockets Still Use Soviet Era Engines канала Scott Manley