Global station duties kept old Royal Navy ships in service

Why did the Royal Navy still have so many ships for Fisher to scrap in the early 1900s?

Because it had worldwide stations to cover, and keeping old ships in service was often the cheapest way to maintain that presence.

Retaining many old ships had a practical basis. A global navy had to maintain presence across many stations, and keeping an existing vessel running was often cheaper than building a replacement. Fisher's later cuts therefore followed from changing the balance between presence requirements and fleet modernization.