why is that rain fall as a uniform spray? is it bcoz clouds act like sieve?

For the same reason if you roll a 6-sided dice 600 times, you are very likely to get close to 100 of each number - extremely unlikely to get anything around 50 of a number. Think of each small area below the cloud as one of the numbers on the dice. There is an equal probability at any given time that a raindrop occupies any of the areas. That is so because of the process involved in raindrop formation.

Raindrops form when tiny cloud droplets which are basically suspended collide to form ones large enough to fall from the cloud. Though it is theoretically possible that the collisions occur such that the drops form a line, random chance of the collisions causes there to be a similar amount of space between them. Not an equal amount of space though - because that also would defy the laws of chance that some will be closer together and some will be further apart. Having exactly the same space between them would be like rolling 100 of each number each time you try the experiment.

Rain doesn't fall as a uniform spray. It's just that the places where it's different are big and the shifts are often gradual. Sometimes, it may just be sprinkling, then all of a sudden, a downpour washes over. Those two sprays are not uniform. Watch a big parking lot during a downpour some time. You will see that the splashes of the ground look different at different parts, and those parts move around.

Clouds like a sieve? Not quite. A raincloud is just that: a cloud of water drops blowing around. Some times the drops get big enough that the wind can't keep them flying and they fall to the ground and we call it rain. Faster wind can keep bigger rain drops flying, so if the wind is strong up in the clouds, the raindrops that fall will be bigger.