With F1 drivers being arguably the best drivers in the world, you would assume then that they would have full control of their cars and not lockup their tires so regularly?
Due to a variety of reasons, it's not as simple as that. F1 cars do not have ABS (Anti-lock Braking System) like road cars or race cars in other motorsport categories. This means that it is up to the driver to prevent the cars wheels from locking up based on experience and skill.
So, why does this happen? When you are driving in your road car and you have to suddenly brake heavily, you'll notice that your body gets flung forward and is held by the seatbelt. Loose items in the car may also get flung forward. This happens because of weight transfer. Everything in the car, including the car itself has it's weight rapidly moved forward becaue of the sudden slow down. This then means that since there's so much weight at the front of the car, the front brakes have a lot of work to do to slow the car down and the rear brakes will therefore have less work to do.
This is why brakes on cars tend to be much more powerful at the front than at the rear. F1 cars have massive carbon brakes that have huge stopping power purely because of the immense forces involved when braking from such high speeds. The problem with this is that a car can transfer weight from side to side as well. This means that when a driver goes through a corner or even makes a slight change in steering angle, then weight will be transferred in the opposite direction.
If you combine all of these actions, then you can get the following scenario:
A driver is approaching Turn 1 at Monza at 340km/h and brakes just before the 100m board. The weight of the car is transferred forward and the driver applies maximum pressure on the brake pedal. This pressure would normally be enough to slow the car down but not so much as to cause lockup. However, the driver then starts to turn in to the corner while braking and this slight change causes the weight of the car to shift slightly to the left. This will then partially unload the front right wheel and since the driver is still applying the same brake pressure to both wheels, then the front right will likely lockup up since the amount of pressure applied is too much for the load on the wheel.
Locking up can also happen when braking over bumps:
If the driver is braking in a straight line approaching Turn 1 at Monza and one of the front wheels strikes a bump, then that wheel can unload and cause a lockup.
This locking up is sometimes referred to as "under rotation" and means that this wheel is rotating a lot less than the three other wheels. Under rotation can result in a tire forming a flat spot which is just the tire skidding along one specific area and causing very high wear. Since the tire is not round any more, this will cause vibration and most likely more lockups since the flatspot will cause the tire to unload under heavy braking.
Finding the balance between maximum braking and not locking up is critical for getting the most out of the car. Drivers will sometimes push the car past the limits during practice to then establish where a safe braking point is for the race. However, as the track evolves over race a weekend because of the track rubbering in and the racing line being cleaned up, these braking points can change. It is then up to the driver to learn how to compensate during the course of the race to prevent lockups while ensuring fast laptimes.
As usual, I tried to keep this explanation as simple as possible without using too many technical terms. Hopefully this goes some way towards helping you have a better understanding of how weight transfer and tire lockups work.