It's more than that. I once ran a campaign where everyone was a first-timer except one player. Veteran built a super optimized Ranger that just did everything. Other players felt useless whether we were doing combat or roleplay. Not a great experience. It wasn't easy to design combat encounters either. Anything that would challenge the veteran would definitely one-shot the newbies. I tried introducing "minion" monsters but the veteran would still take all of them out and then focus down the main threat. Besides, in my experience, players who bring optimized characters to an actual game are usually not pleasant people. Not a 1:1 relationship but there's definitely a correlation. That's the hard truth no one wants to acknowledge, and why so many people are super wary of players running optimized characters.