It's an interesting question because our current laser guns look more like giant flat panels that fire invisible beams of heat at airplanes and torpedoes.

But those are aimed by computers. People need to use sights to aim. A longer barrel gives a more accurate sight and better control of the gun. It also allows the weapon to function like a bat in close fighting and could hold other attachments like grenade launchers or backup conventional rifles for when laser just won't work (like the Chinese mirror armor they developed in response to USA laser weapons).

User Expectation and Skeumorphic Design

The floppy disk 💾 is used (less so than five years ago, but used) as a save icon, even though outside of old industrial machines and nuclear missile silos, no one uses floppy disks. Your phone makes a shutter sound when you take a picture, despite not having a shutter. Electric cars are designed to look like gasoline vehicles in many ways that are utterly unnecessary, because it’s what people expect, and when something looks the way people expect, it makes them happy and comfortable and more likely to award your Laser Gun Company with the government contract for one million Pew Pew Guns. So your industrial designers make it look like a nice old fashioned projectile-throwing rifle, even if it doesn’t strictly need to.

I guess it is just for readers/viewers to be able to recognize in a fraction of a second that the character is holding a weapon that can hit at a distance.

To justify this would be so that military personnel doesn't need to adapt to a new way of fighting and can be even more efficient than before without the required formation time.
Then you still need batteries, plasma ammunition, electronics to handle the energy release, attachments and so on that make is as big as a classic gun.

Cooling system

Longer, mean more cooling, mean more heat can be dissipated, mean more powerful laser / you can use it for a longer period of time

That might be unrealistic but in a sci-fi world, why not (imagine a new metal/cooling technology to miniaturize everything)

A real futuristic personal laser weapon would probably sit on someones shoulder and the aim point would track whatever the user was looking at. It would not look like a gun at all.

To aim a weapon accurately, a long barrel is helpful. You also want to have some heft to dampen jitter (Photographers know that a light camera is harder to hold steady).

As well, you need some place to put all of the tech, even if you don't need a long lasing cavity you'll find some use for the volume. A short but thick device would be much less practical, for aiming as well as for carrying.

Of course the validity of all of these reasons depends entirely on the author's whim in a scifi setting. You coud have a short, giro stabilized hand gun with computer assisted aiming and nano components if you so please, but who wants to ruin a good trope?