Americans "Obsession" With Germs
Was born out of very good reasons.
Before science revealed the exact cause, transmission, and prevention, there were many, many diseases that ravaged the population. Also since this was before antibiotics, sulfa drugs, and other modern day "miracle" cures, a simple cut or scrape could turn into an infection that lead to death.
Polio
TB
Thyphoid Fever
Influenza (especially the great pandemic of 1916)
And so on.
The best defense was thought to be a good offense. Hence proper Amercian housewife saw it as her moral duty to protect her house and family from disease. This meant keeping all and sundry clean as possible.
When it came to laundry, Europeans favoured boiling their wash over "Eau de Javel" or other forms of chlorine bleach for a very simple reason. Most all linens bed, body, personal, household and so forth was just that, made from pure linen. Chlorine bleach will do serious damage to linen fibers, so it was avoided at all costs. The quality of European housewives, housekeepers and the rest of the wealthy went to great lengths to make sure their fine linen was not subjected to bleach.
OTHO cotton, which grew in the United States, and was abundant, replaced linen for most if not all purposes. More so after the southern states began to grow and produce the stuff en masse via slave labour. Cotton while also a cellouse fibre like linen, can withstand chlorine bleaching.
On both sides of the pond, boiling was done to laundry AFTER it had been soaked for several hours and or soaped. In short it was done to shift soils without all that beating and scrubbing. Boiling also killed vermin and germs, but we're not on that right now.
When washing machines came upon the scene, boiling started to die off as part of routine laundry in the USA. However Europeans still had all that linen, and favoured high temperatures for washing regardless. When Henkel invented Persil with oxygen bleach, it cemented high temperature washing in Europe until rather recently. The favoured method of bleaching in the wash for Europeans was with perborate bleaches. Sodium perborate will not begin to release oxygen until temps reach about 60C, and the action is greater the higher wash temperatures go. Hence all that boil washing.