Kyrsten Sinema's gumming up the works on Joe Biden's agenda, while frustrating and/or infuriating, is not all that surprising by any stretch. The reason for her obstruction is glaringly obvious: she's aiming for Biden's job. That's right, Sinema wants to be president.
The fact is that Sinema is the last real dying breath of the corporatist New Democrats. Think about how much time she's been spending with big-dollar donors instead of working to pass legislation. These big-dollar donors include big business, trade groups, etc., and they want Biden's agenda killed as it's too closely aligned with that of congressional progressives.
Sinema is much easier to control by comparison as she's been a "Team Player" by moving the Democratic Party further to the right. Under her leadership, the Democratic Party becomes the oppositionary far-right party to Donald Trump and his lackeys in the Republican Party. When that happens all bets are off.