They do year round school in some inner city areas.
My own local elementary and high school districts offer free school meals year round - non perishable items are picked up at the schools once a week in the summer and winter breaks.
All the schools in my local area give out summer math packets to complete. Elementary school students ta are taken to the local libraries to get library cards and a tour of the facilities, with the hope that they'll be able to use them. The high school requires summer math packets be completed and handed in by the first week of the school year, and there is assigned summer reading too, with reflections required in the first week of the new semester too.
Learning loss? Depends what you consider falls into the category of 'learning'.
American schools tend to have a holistic idea of what constitutes learning - it encompasses social, emotional, and physical development, not just mastery in the academic sense.
Hence all the emphasis on extracurricular activities, volunteering, etc. Hence also the myriad programmes and facilities available to kids in most city and suburban areas, from church camps to park district enrichment offerings to camps focusing on sport, art, performing arts, music, etc. Hence also the emphasis on youth summer employment - the local big city hopes to employ about 40k teens in various roles this summer.
The long summer break (which happens in most other countries in the developed world, fwiw,) gives kids the chance to grow and learn in a great many ways.