Unfortunately, around here, most of the self-checkout "lanes" are explicitly marked as "X items or less" express lanes. If you're doing a full shopping, they don't want you using them. (This seems particularly stupid at one of the stores, where they have about a dozen self-checkouts, half marked 14 items or less and half marked 20 items or less, and literally every time I'm there, at least half of them are unused. Fortunately they also have a lot of manned regular checkout lanes.)

Furthermore, because the expectation nowadays is that the cashier will bag the groceries, too, the checkout infrastructure is very much set up to support that and only that model: rather than having a short belt after the cashier to send the groceries to a bagging area, the cashier has a couple of bag slots right in front of them, and a tiny island behind them to put your bags on, along with any items that they need to hold onto to bag later (eg, chips, eggs—things they don't want to put under other things). So even if you wanted to bag for yourself, it would make it much less efficient and more awkward for the cashier.