(Not to mention that many people either refuse to wear masks or don’t wear masks properly covering nose and mouth).

If you lift lockdown, lots of people get infects. It takes a few days, maybe a week to show symptoms bad enough to go to a doctor for testing. This is the point in time when cases begin to go up. Imagine if you lifted the lockdown for a day and went back.

One to two weeks after that, people will get more seriously ill, requiring hospitalization. Initially this can be treated easy at a (presumably) low-load hospital with no issue, the death toll is low.

One more week and you have the majority of cases in the hospital, this is where most people will die. A month after you lifted the lockdown.

Now imagine not locking down after that one day. After 1 month, you're at the peak of Day 1 infections, the next day the peak of day 2 infections etc. It compounds fast and can overwhelm a hospital.

Once a hospital is unable to care for all patients properly, triage will be applied. People with good recovery chances will receive treatment, those who are risk groups will not.

That's when your death toll will explode very suddenly.

Death toll is a trailing indicator (IIRC, it tends to be about 10 days from infection to symptom onset in symptomatic cases, a little bit longer to hospitalization, a few more days to death where it occurs, and that's getting longer because of better care protocols with more experience, so the degree to which death is a trailing indicator is increasing), we've gotten better at managing serious cases (which is better for ventilator capacity since we are using less per case, but worse for overall hospital capacity, IIRC, because we're using more bed-days per hospitalization.)

The fact that cases over time are skewing younger also depresses the death rate but that's somewhat misleading as to impact, because seriously ill younger people are less likely to die but also more likely to have long hospitalization and post-hopsitalization convalescence, and that's only considering immediate consequences and not the increasing array of significant delayed health consequences that are being identified.

To slow the spread so that our medical facilities have the capacity to handle the sick, and we don't have to let people die due to lack of healthcare resources.

First, you lessen the load of the healthcare system. If a lot of people get sick at the same time, hospitals would be swamped and more people would die because care is impossible.

Second, you can bring the disease from an exponential growth to sub-exponential, potentially containing it mostly (this is what happened for example in Germany, where tracking individual infections is now possible again).

Third, you buy yourself time until a vaccine is available. Even if no vaccine is going to appear in the near future, as long as you slow it down long enough (1-2 years) from reaching saturation in the population, that's lives saved.