For centuries it kept the Americas from being discovered by the people of Europe.

by是“被”的意思
keep sb from doing sth是固定搭配 解释为“阻止某人做某事”
你说的be discovered by 肯定不是固定短语啦~

by 在这里解释“被“
be discovered by 被。。发现

by 为。。。所发现，介词

几个世纪以来，每周都没有为欧州人所发现。