Because it's DJI... a Chinese company trying to determine the locations and classifications of airfields all around the globe using outdated and geographically incorrect databases of unknown origin. Even the FAA's own database can be wildly inaccurate for smaller and/or older airfields in the US. If the governmental agency in charge of all US airfields can't keep them straight, there's not much hope that a drone company (with no real incentive to get all of the data correct) will be the most accurate resource for airfields spread out across the planet.
II would argue that it is wholly in DJI's interest to get the most accurate data from local authorities possible and update it on the fly or at very least asap. Ultimately they can argue with any local authority that they are using their supplied info to the letter and therefore put the onus of responsibility for any problems due to zoning errors back on the supplying local authority. If drone-flying problems arise specifically because of DJI's erroneous data, when local and updated data is freely available then who gets the finger pointed at them?
A strong argument from any government to ban or restrict drones is that there's a lack of interest from the drone makers to highlight correct information to users as regards no-fly zones, etc. As such local legislation and anti-drone media coverage is increased, reducing potential consumer sales as people become more and more disenfranchised with the whole idea of drone ownership and usage.
Indeed, one of the main reasons I purchased, finally a great drone, was because I do live in a completely safe zone with free flying all round. Now, to find I am actually in a warning zone erroneously, according to DJI, is a fly in the ointment. But I can imagine many potential customers being put off wrongly, even purchasing a drone because of wrong info.
With the many billions of USD involved (reading that the drone market for 2017 was 17 billion USD with DJI controlling 70% of it), it's wholly and completely in DJI's interest to invested a tiny, miniscule, fraction of their profits into updating regularly their zone information in accordance with local data.