Many of you would gain from the history of ICAO documents
One of the wonderful things that came about as a result of the ICAO Convention of 1944 was the consultative groups that were formed to develop the Standards and Recommended Practices for world aviation as empowered under Article 37 of the Convention. Those consultative groups not only thrashed out rules within their own areas of expertise but also ensured that the world was provided with one of the most cohesive and consistent set of guidelines for everything aviation.
400 feet appears in many of the ICAO Annexes - it is a standard level for obstruction charting, aircraft performance, instrument approach design, obstacle limitation surfaces for aerodromes, etc. There would have been many inputs into the original derivation of the 400 ft from all of those areas and undoubtedly many conflicting imperatives. I have no access to the historical data that would record whether the FAA adopted the ICAO figures or vice versa, but suffice it to say that 400 ft is a universal standard for control of the design of aircraft and the associated infrastructure.
ICAO is as much about an economic level playing field as it is about safety - lowering the minimum acceleration height would change to power requirements for aircraft (less), perhaps reducing other related performance requirements while substantially increasing the costs of rewriting all the AFMs, conducting airport surveys, making charts, installing obstruction lighting, maintaining obstacle control, limiting noise pollution and other environmental consequences, etc. And of course, each Contracting State would need to ratify and codefy the changes to an agreed schedule and....
I for one do not want less installed power or engines with shorter power ratings limits nor do I want to accelerate closer to the terrain than what is already a litle daunting if you watch the oibstacles coming at 1.6%.