Because it was designed for modern hardware rather than the 16-bit Neo Geo MVS or AES, there is no official Neo Geo ROM for this title.
To understand why you cannot find the ROM, consider the hardware limitations. The Neo Geo MVS ran on a 16-bit Motorola 68000 CPU at 12 MHz. The Nintendo DS ran on two ARM processors (ARM9 at 67 MHz and ARM7 at 33 MHz). metal slug 7 neo geo rom
Because it was built for the DS, it utilized features like the dual-screen for a real-time stage map, which the original Neo Geo hardware could not support. Why There is No Neo Geo ROM Because it was designed for modern hardware rather