Consider that senior developers are busy and aren't going to make time to prep for interviews that lead to a job landing them a $15k-$20k bump (before taxes). Together with all the other intangibles (culture, nature of project work, career paths) it makes jumping ship difficult to gauge.

Three suggestions - 1) raise the compensation for seniors 2) offer more interesting projects/products 3) use a different interview process for seniors, they hate algorithm quizzes

Most software companies just churn out database-backed MVC apps. After 5 years of that any good developer would be bored.