In order to be considered an apostle, one had to have been personally taught by the Lord and have seen Him in a post-resurrection appearance. Paul did see a post resurrected Jesus, but he was never personally taught by Jesus before His resurrection, right? Couldn’t a person today say the same thing? They could say that Jesus came to them in His post-resurrection body and taught them revelation pertinent to us today.
Paul said he didn’t receive the gospel he preached from any man but by revelation from Jesus Christ (Galatians 1:11-12). After his conversion on the Damascus Road Paul went directly to Arabia (Galatians 1:17) where many scholars believe this happened. Later, when he want to Jerusalem, he spoke with James, Peter, and John who all confirmed him as an apostle to the gentiles, just as Peter was an apostle to the Jews (Galatians 2: 6-9).
A person could make a similar claim today, and in fact, some do. But none of them can receive the confirmation that Paul received.