When you release the clutch smoothly in a car's first gear without accelerating, the car may still move forward due to the engine's idle power. The idle power is generated by the engine even when you're not actively pressing the accelerator pedal. This power is sufficient to overcome the car's inertia and allow it to move forward slowly.
On the other hand, a bike typically requires more acceleration to start moving. In most motorcycles, the engine's idle power is not strong enough to propel the bike forward from a standstill, especially in a higher gear. In order to get a bike moving, you usually need to apply some throttle to increase the engine's power output and generate enough torque to overcome the resistance and start moving.
It's important to note that the specific behavior may vary depending on the make and model of the car or bike, as well as their individual design and specifications.