Thats not true, the Scalar Efficiency SIG is currently working on such an extension.
See this spreadsheet of discussed instructions: https://docs.google.com/spreadsheets/u/0/d/1dQYU7QQ-SnIoXp9v... and charter: https://github.com/riscv-admin/riscv-scalar-efficiency/blob/...
Qualcomm prefered a strict 32-bit instruction set, with potentially 64 bit naturally aligned instructions. RISC-V is designed for 16, 32, 48, 64 bit instructions that are 16 bit aligned, and retroactively changing that wouldn't have been a good decision. Both sides of the argument agreed that both options are resonable and don't hinder high performance designs.
Qualcomm seems to have accepted this now, as they e.g. proposes 48 bit instructions with larger immediats.