No, plenty of private companies are profit driven

"Because" often doesn't make sense in the real world where anything that happens is the sum total of multiple factors. Especially a trend as broad as enshitification.

Being publicly traded seems like a contributing factor, but a sole owner can still enshitify through greed or incompetence.

I would guess that market consolidation and low competition are more of a factor.

Reddit was shit before it went public or even before they were talking about making it public, so no.

The company I work for has acquired a number of small companies over the years; the result has been a mixed bag. In one case, the original product and employees were dropped completely, only retaining the IP. In a number of other cases, the original teams and products were kept intact with cross-over between products plus a huge boost in funding and customers over the years. In most cases, the companies were absorbed into existing management structures and the employees and technologies deployed inside the existing product line, sometimes with a few things that didn’t match the company strategy sold off or spun off into their own company.

Personally, I consider all the acquisitions except the single case where everything was abandoned to be a success; in that case, the exec in charge of acquisition was made redundant when everything else shut down.