really confused with getting added onto car insurance

I just got my license and my mom has two cars and she wanted to add me as a “occasional” driver onto one and we planned for it to just be mine to build my own reputation for when i have my own insurance on my own.

For some reason they dont allow that..? they said since we have two cars and two fully licensed people that i need to just go onto my own insurance immediately and im expecting around 500$ CAD a month. Is this normal? i really wasn’t expecting this and i dont think i can even find cheaper than that at other insurance places.