The reason for that is that most memory bandwidth bumps come with new memory generations. For example an early DDR4 platform (e.g. Intel Skylake/Core iX-6000) and a late one (e.g. AMD Zen3/Ryzen 5000) only differ by 1.5x as well, typically.
The same trend is visible in GPUs: for example, my RTX 2070 (GDDR6) has the same memory bandwidth as a 3070 and only a little bit less than a 4070 (GDDR6X). However, a 5070 does get significantly more bandwidth due to the jump to GDDR7. Lower-end cards like the 4060 even stuck to GDDR6, which gave them a bandwidth deficit compared to a 3060 due to the narrower memory buses on the 40 series.
The same trend is visible in GPUs: for example, my RTX 2070 (GDDR6) has the same memory bandwidth as a 3070 and only a little bit less than a 4070 (GDDR6X). However, a 5070 does get significantly more bandwidth due to the jump to GDDR7. Lower-end cards like the 4060 even stuck to GDDR6, which gave them a bandwidth deficit compared to a 3060 due to the narrower memory buses on the 40 series.