They literally didn't need to. There is no fundamental difference between having a 5/9 majority of Federalist Society ghouls that will never fail to hold the party line, a 6/9 majority of Federalist Society ghouls that will never fail to hold the party line, and a 47/50 majority of Federalist Society ghouls that will never fail to hold the party line.

Not packing the courts is also tactically and optically a better option, because when you inevitably lose in 2020 or 2024, a possibility they already had planned for in the handbook, you don't immediately have a democratic Congress come in hot off the tails of 75% of American voters being livid at you for overturning Roe, Obergefell, and fucking Plessy v. Ferguson, immediately pack the courts with a thousand 30 year old clones of Merrick Garland that can live another 60 years, and then pass a law permanently keeping the supreme Court at a size of 1050, and guaranteeing dem judicial success for generations.

Tl;dr: They're evil, not idiots. It doesn't matter how big your majority is when all your justices are ghoulish and you've guaranteed that all even moderately high profile conservative judges on any level in the past 50 years have been equally excited to eliminate abortion, gay marriage, and reinstitute Jim Crow laws nationwide.