HE does it like that to protect his smaller bankroll. Its true when you bet higher you can make more but you can also lose everything. Its different for everyone.
With bigger bankroll though, you can build many sessions which give you many chances of hitting your profit goal, be it daily, weekly, or monthly profit goal
But when playing with bigger bankroll one can lose a lot of money very fast without even realising it because they may be continuously betting thinking they got a big bankroll but we all should remember we can't beat house bankroll to win at the end. I prefer to gamble with some fixed bankroll to limit my losses on a daily basis.
If you bet a strict % of your bankroll, you won't lose money quickly. I don't think you know how proper bankroll management works. If he bets 5% of his bankroll on every play, then he will lose less and less after each loss, minimizing his losses. But if he keeps winning, then his bankroll with get exponentially bigger