On the contrary, it's for your safety instead (the customer) that the manufacturers do that. It's almost important that very high powered flashlights like the X50 or MK38 have a built-in battery pack to properly and safely handle the charge and huge discharge requirements, as well as cell balancing and so on.
Tactical_grizzly has done a good teardown of a X50 battery pack: https://budgetlightforum.com/t/-/69378
For a variety of reasons, having such a high powered flashlight with individual user-replaceable cells becomes very problematic. There is a discussion of some of the issues on my MK38 thread. https://budgetlightforum.com/t/-/69119 . Even flashlights such as the D18 which puts 3 user-replaceable high-powered cells in parallel is arguably (and I know an unpopular opinion) borderline dangerous as a consumer product (but yes I understand that it's fine as an enthusiast item). Any accident caused by improper use of the flashlight, even by the user, can be a big liability for the company since it is their responsibility to engineer a safe product.
It's definitely possible to design a decently powerful flashlight which accepts 3 individual cells, and has all the appropriate electro-mechanical protection, and also supports USB C charging and powerbank functionality, but you'll be looking at a $200-300 product at that point, not close to the $100 price-point of the D18 or similar. The MK38 comes in a user-replaceable version with no USB C charging, but I do hope it has the right protection inside. If you compare the integrated and non-integrated versions of the MK38, an extra $60 gets you a completely different machined body, a decently implemented USB charge and powerbank circuit, 3 very decent 21700 cells, and (hopefully, I haven't verified this yet, but high likelyhood) an integrated BMS. For a product that likely sells only in the hundreds, the profit margins are likely not very high and I think it represents OK value.