You're right that it doesn't have to be that way, but there are good reasons it's remained so. Code re-use in ES is fiendishly difficult because every system is a special little snowflake. Moreover, ES teams are fantastically understaffed relative to the complexity of the systems they own.

Because nothing works by default on ES systems, you have to implement it yourself most of the time. If you can clear that hurdle, your company is unlikely to be open source friendly. If you can clear that hurdle, people on other teams won't have time to adopt it for their systems, so it won't spread. If you can clear all of that, all you've won is a single tool. In the meantime, the rest of the industry as written half a dozen.

There are things you can do here, but... yeah.