A Word document isn't just text and some formatting sigils. Editing isn't just appending bytes to the end of a file descriptor.
It's a huge memory structure that has to hold an edit history so undo and track changes works, the spelling and grammar checker needs to live entirely in RAM since it runs in realtime as you edit, and the application itself has thousands of not millions of allocated objects for everything from UI elements to WordArt. The rendering engine needs to potentially hold a dozen fonts in memory for not just the system but any specified but not immediately used fonts from the base document template.
It's not like Google Docs is any lighter on RAM than Word. Features require memory. Fast features are usually going to require more memory.
People can use AbiWord if they want a much slimmer word processor. They could also just use nano and Markdown if they wanted even slimmer. But a lot of people want document sharing over the Internet with track changes, grammar checking, and the ability to drag in and edit an Excel spreadsheet.
The features used in native software follow a bathtub curve. But not just one but several. No two groups necessarily use the same sets of advanced/uncommon features.
