Because... the TNT2 M64 is a very slow graphics card, with very low memory bandwidth due to its 64 bit memory bus and slow video memory.
It will depend on the game and your personal preference, but there will be very few games that run well (by modern standards) on the TNT2 M64 at resolutions higher than 640 x 480.
Even the TNT2 Ultra will rarely (never?) hit 60 FPS at 1024 x 768 x 32 (it usually barely manages 30 FPS at this resolution), so you can definitely lower your expectations when it comes to the M64. 😀
If you want a big boost in performance, my advice is to search for a GeForce 2 MX / MX400 with a 128bit memory bus, or, even better, a GeForce 2 GTS, if you can find one for a decent price. Another very good (and still pretty cheap) option would be a GeForce 4 MX440/460 (again, make sure that it's an actual 440/460 with a 128bit memory bus).
2 x PLCC-68 / 4 x PGA132 / 5 x Skt 3 / 1 x Skt 4 / 9 x Skt 7 / 12 x SS7 / 1 x Skt 8 / 14 x Slot 1 / 6 x Slot A
5 x Skt 370 / 8 x Skt A / 2 x Skt 478 / 2 x Skt 754 / 3 x Skt 939 / 7 x LGA775 / 1 x LGA1155
Current PC: Ryzen 7 9800X3D
Backup: Ryzen 7 5800X3D