> Then why is getting into M.S programs much easier?

Because domestic undergrads applying to a research-oriented grad program are advised to apply directly to a PhD program.

MS/MA programs are expected to remediate deficiencies at the undergrad level, and most undergrads will take 1st year MS/MA classes as part of their senior year doing a bachelors. Alternatively, the MS/MA is awarded if a PhD candidate failed Quals.

Outside the US, the expected path is BA/BSc/BEng/BTech to MS/MA/MTech/MPhil to PhD.

In the US, the expected path is SB/AB to PhD and MS/MA if you fail quals or are looking for a professional program to remediate coursework deficiencies.