Ventilation of the lungs isn't like filling a balloon with air. The use of PEEP, positive end-expiratory pressure, isn't how the lungs function usually.
Inflation of our lungs occurs by developing a negative pressure in the thorax by contracting the diaphragm and raising the muscles between our ribs. This both keeps the air sacs (alveoli) open and inflates them, producing airflow. It is the negative pressure compared to the air surrounding us that pulls air into the lungs. The air moves down the pressure gradient.
Applying a pressure inside the air sacs can cause damage to the delicate lining leading to pulmonary oedema and worsening hypoxemia that can prolong mechanical ventilation, this can then lead to multi-system organ dysfunction, and increased mortality.
Not all COVID-19 patients in ICUs have air sacs congested with fluid; some have problems because of fluid in the spaces between the air sac and the capillaries making oxygen uptake difficult.
The ventilation required is much more sensitive. This is why scurrying around for ventilators isn't the answer. The right type is needed.