This is such a common occurrence in GCSE essays and it loses marks because it weakens the analytical precision of the essay - an aspect which should be at the core of all good essays. Look at these two examples - one a generalisation and one not - and the reason might become clear:
1. 'In Steinbeck's 'Of Mice and Men' all of the characters are poor and downtrodden.'
2. 'In Steinbeck's 'Of Mice and Men', the poor and downtrodden characters are assigned to the bunkhouse, whereas the Boss and his son live in their own houses.'
Can you tell which sentence is the stronger because it is not generalised and which sentence would gain more marks? Try this again:
3. 'In 1930s America, people were poor and struggled to find good, well paid jobs.'
4. 'Many of the ranch-hands in Steinbeck's 'Of Mice and Men' are depicted as poor.'