Because both parties are scared eventually the other party will be back in the majority.
https://en.m.wikipedia.org/wiki/40th_United_States_Congress#...
https://en.m.wikipedia.org/wiki/74th_United_States_Congress#...
https://en.m.wikipedia.org/wiki/76th_United_States_Congress#...
https://en.m.wikipedia.org/wiki/89th_United_States_Congress#...
Maybe you're right, but people always say that. And they've always been wrong.
As one plausible mechanism: the economy stalls / inflation accelerates and people blame the party in power.
In the 2026 senate election, the Dems could absolutely flip Maine, North Carolina, and two others; maybe Alaska and Ohio.
If Elon Musk makes good on his threat to try to take out sitting GOP senators, that splitting of the vote could mean the Dems pick up a few more as well.