Google re-crawls every URL they have ever seen at least several times per year.
These "errors" are normal. Google needs to see that the pages have gone, and they will check several times.
You have several choices: - adopt some of the URLs by placing content there or by redirecting those requests to a page on the new site that closely matches whatever was on the old page. - completely disassociate the new site from the old URLs by returning 410 instead of 404 for those requests. - do nothing and let Google figure out on their own timescale that the pages returning 404 no longer exist and aren't coming back.
Do NOT redirect the URLs flagged as errors to the root URL of your site. That would be a technical disaster.