Lowes does not carry any Ryobi tools in stores because the brand has a contract with Home Depot, one of its chief competitors, to sell products exclusively at its stores.
This isn't uncommon; the two retailers have lost brands to each other in the past. This includes Milwaukee, which shares parent company Techtronic Industries with Ryobi.
Although Ryobi is sold exclusively at Home Depot, it doesn’t account for online purchases. You can buy tools from the brand at places like Amazon or Direct Tools Factory Outlet.