7.5 for Zenimax is super cheap already, Bungie was reportedly asking for 3 billion for one IP, Warner wanted 4B for WB Games with no IPs. Microsoft got Zenimax, all of their IP (just Fallout alone is huge) and 8 studios - mainly AAA studios - for 7,5B. Why the low price? Because Bethesda had been making flop after flop for the last 5 years: Wolfenstein 2 flopped, Youngblood flopped, Rage 2 flopped, Fallout 76 flopped, Doom Eternal has underperformed, Prey flopped, Dishonored 2 flopped, Dishonored DOTO flopped...
They probably saw that a huge game every 5 or 6 years (Fallout/TES/Starfield) would not save them in the long term if all their other games flopped.