How to Draft a Guard and Defensive Lineman

Elaina says, I have a question that feels very dumb, but no one has been able to give me a good answer. The too long don't read is what is the point of drafting guards when you could just draft tackles? She's like, I get that guards and tackles of different roles. But my general understanding is it's harder to bump a guard to the outside than it is to bump a tackle to the inside. Why not just bring in someone that can probably play both? And is it limited by size? Just have big people.