Which is in direct contrast to the UK or the Scandinavian monarchies. The Netherlands hasn’t had a monarch actually die on the throne since 1890 (also the last they had a king; Wilhelmina & Juliana both abdicated in favour of their daughters. There’s been speculation that Queen Beatrix might break with tradition and reign until she dies.
Belgium doesn’t really have a tradition of monarchs abdicating as a form of retirement. Leopold III was the only Belgian monarch to abdicate and that was the result of a pretty severe constitutional crisis.