As far as I know there are kind of 3 reasons:

• Your sink pipes are smaller than your toilet pipes (by ALOT) which means that they clog easily
• In some places your sink pipes and your toilet pipes go to different places. For example in some places the greywater (water from the sink) is let out in the open (which is fine as it is mostly just dirty soapy water) while wastewater (poopy) goes to the sewer.
• Poop is usually not sticky and fatty while foods can be (imagine if you poured flour down the drain, it will become a sticky dough that will clog it)