Hundreds of megabytes is not that much, people can fill that up easily without sending large emails. Gmail can't predict the sizes of the emails you'll receive

Hundreds of megabytes is not that much, people can fill that up easily without sending large emails. Gmail can't predict the sizes of the emails you'll receive in the future. Maybe all your attachments are small but you'll receive large emails. Warning you when you only have a dozen MB left could well be too late.