It is because the majority of cases are misconstrued as a common cold or flu, and there are not enough testing kits available, that nobody can really know whether they actually DO have a common cold/flu (which has a 0.5% mortality or lower) or if they have COVID-19 (with it's 3%+ mortality). Therefore, community spread is inevitable if life goes on as usual. The majority of the population will continue to work through a cold and possibly a flu, spreading it to everyone else in their workplace, or students in school, going to social gatherings and so on.
One thing COVID-19 has demonstrated is that it is very contagious and while not particularly deadly (compared to SARS 10%, MERS 50%, Ebola 90% and so on) the fact that it can disseminate to so many people over a short rate of time, and has several times the death rate of the flu (although the number may be overblown because we don't include milder unknown cases), it can still be devastating to our healthcare systems. We already have 30,000 - 60,000 people die each year from the flu, despite vaccination attempts and only a 0.5% mortality (somewhere less than 1% and some say as little as 0.1%).
Now we have a virus that is known to be more deadly than flu by at least several times (estimates vary from 2-3x to over 10x) and absolutely no vaccine, no immunity, and added ON TOP OF all the other cold/flu viruses still out there. So we are battling multiple viruses and particularly nasty ones at that superimposed on each other. Yes the majority of people will be fine but those that aren't will completely overload our healthcare systems. To prevent the collateral damage and keep healthcare systems intact for all the other problems people face, these somewhat draconian measures are the only way to help delay/prevent the inevitable... so at least things "ease in" a little more slowly. Will it cause the "case growth rate" to plateau and reach <1.0x, probably not. But lower rate is better than higher.