Potatoes, pasta and boiled jasmin rice are all starch and do not taste much by themselves. However I would not serve tomato sauce nor olives to the rice. On the other hand I would not serve potatoes or pasta with soya sauce.  Why is that?  Also oven roasted potatoes may be good with ketchup and even mustard. So oven roasting "transforms" the starch in the poatoes somehow.   Likewise frying the rice (pilaf) make it "fattier"/"heavier" than boiled rice and the range of what you can add to the rice increases. In this case fat is added but I think it is the "roasting" that is the main thing.  So to me it seems that there is some property of the starch. For lack of better words I will call this roastedness.   Roastedness(potatoes) > Roastedness(pasta) > Roastedness(rice).  Oven roasting or frying increases roastedness for all.  What is this property I call roastedness?   May pH have something to do with this? Tomatoes are acid. Rice is slightly acid. Potatoes are slightly acid except for the skin which is alkaline. On total potatoes are therefore slightly alkaline? Soya is alkaline. Oven roasting or frying potatoes produces acrylamide which is alkaline? Combining alkaline (bitter) and acid (sour) taste good. 
Because you are from a specific country with his own culture.  It uis quite normal for example in Italy to have a Risotto with tomato sauce, olives and tuna maybe (and even capers).  It is also quite normal to have noodles (some of which which are exaclty the same type of weath as pasta) with soy sauce, in a soup or in a stir fry.  The properties of the roasting that you are talking about are not related to fat (not directly). It may seems that the reason is the fat, but roasting involves fat and salt and maybe spices that give more taste, therefore making it more sutable to mix with other ingredients as the taste will not be lost. You wouldnøt eat boiled potatoes with mustard because they will taste mostly of mustard, but with a roast you'll get a better balace of flavours. But if in your culture that is an acceptable taste it will still be used.  So the final answer is definetely cultur based 