Sure a rich person might have options to just lump sum into bitcoin and/or even to frontload their investment into bitcoin, yet they still might choose to buy bitcoin in increments from their income as it comes in rather than going through some process to allocate a bunch of money into bitcoin at once.
For sure there can be a convenience factor to DCA rather than to lump sum, yet the rich person has options, and the poor person might rarely have options to lump sum unless he happens to get a bonus at work or some kind of an irregular event causes him to have extra money that he had not expected.
DCA gives us abilities to adapt our bitcoin buys to our cashflow situation, yet we are not trying to buy bitcoin at a higher price, since we probably would prefer to buy on the dip - however, there is no way to know if a dip is going to come or not, so there might not be much if any value to hold back money for buying dips that might not happen, even though it is likely that the rich person would be able to do that because the rich person has resources.
I think wealthy people still choose a gradual approach to investing because they also have responsibilities. Even though they have easy access to existing facilities, they won't do it haphazardly. They also know that carelessness can lead to significant losses. Therefore, they still choose to invest in stages even if they have sufficient funds to invest in Bitcoin.
It's true that DCA offers convenience, but everyone has their own principles especially when it comes to money. They're certainly more cautious with every expense, reasoning that nothing is accidental whether they're poor or wealthy.
Everyone wants a bonus, but many poor people have overly high expectations. This is due to the desire for extra money. It's perfectly natural for them to hope for a bonus as an additional benefit for those who are poor.
This DCA makes it very easy for parties to make purchases, because each person's income is different, so when parties who are less in terms of income can still make purchases even in small amounts, especially since there are no rules where this DCA is only aimed at parties who have a lot of income or money, so this DCA is one strategy that is very helpful and makes it easier for anyone who wants to buy Bitcoin.