Because unless you have serious problems with nutrient malabsorption and therefore problems related to the gut, then you are not eating and exercising properly,
The speech is quite simple but the fact is that it is complex.
I’ll give you some input
Make sure you know how much muscle mass you have now, if you can’t do the Bioimpedance analysis then do this, check how much you weigh and based on that value you have to subtract the weight of the blood (about 5 liters, of the brain about 1 or 1.5 kg) and the weight of the skeleton (if you are a tall boy between 1.70 and 1.80 and weigh about 70 kilos you have to subtract another 14 kilos) about 20% of the weight.
You will get a value. let’s say that you get 60 kg, that’s that and more or less your mass, now you have to consider eating about 2 grams of protein per kilo so 120 grams. Here make sure you assimilate a mixture of animal and vegetable proteins (these in the majority) during the day and keep in mind to eat a maximum of 30 grams per meal otherwise you risk putting on fat. Here you can find even more complete information that will guarantee you the improvement of strength and muscles
Follow an exercise schedule that varies between squats
Push-ups with backpack behind the shoulders and abdominals with weight on the arms where you exercise by lifting the torso and weight on the legs where you raise and lower your legs from lying on the ground. All to start with 3 sets of 20 but every month you need to gain weight