To answer the title question, I'd say it's close to the problem, but not quite: It's not that there's too many/few encounters per day, it's that the awkward short rest mechanic along with the general lack of tools for time pressure (and noncombat encounters) means that holding up this encounter day is exceedingly difficult. 1 hour is a long time for a break and often in areas where an 8 hour rest is also possible, discouraging short resting most of the time (The gritty rest rules usually don't have this problem, however). The game also doesn't reward pushing on for more encounters like 4e does, either.

That said, handling encounter days has the additional caveat that not always do player want (or in fact, are able to) to complete the encounter day and rest early. While abusing long rests is a problem, so is forcing the group to march forward into a guaranteed TPK due to lack of resources. And interrupting a long rest will only make the group want to long rest even more.