My experience is completely the opposite. I am constantly awed by how many people are willing to help and by how many are willing to use their English with me.

The difference is probably in approach. 5% of the passers by in a crowded street is a heck of a lot of people, but expecting to identify a member of this 5% by approaching someone randomly obviously leads to (from the comment above) amazement when someone who isn't bilingual is selected as a target.

If you set yourself up as a target - for instance, pull out a map and look puzzled as you attempt to decipher it - those who would choose to assist you or/and who feel comfortable using English are more likely to extract themselves from the other 95% of passers by and approach you.