Consider it this way. Ubuntu developers look to providing the best experience by default. If there’s something that can be done to improve the experience for users, it doesn’t make sense to put it in a program that users can run; it makes more sense to just make the system install with that change in place already.
There may be specific things that you believe aren’t necessary for you that use resources, but we have to make something that works for everyone. For example: accessibility tools are available by default (I believe). It wouldn’t be appropriate to ship a program that just removes them.
There are Ubuntu flavours that focus on being light, and there are other distributions that go to longer lengths still. If you really want a stripped-down but customised installation, then these may be better options for you.