It is likely that you find them harder to read mostly because you are less habituated to them.For many of the older generations, who have spent their childhood reading thousands of books printed using serif fonts, at a time when there was no easy access to computer terminals, serif fonts are easier to read.
In general, the readability of a typeface is determined more by other features than by whether it is has or it does not have serifs.
The sans serif typefaces have appeared first after the Napoleonic wars, as simplified typefaces, suitable for low-quality printing used for titles or advertisements that should be readable from a distance.
Having simplified letter forms, sans serif typefaces remain preferable for low-resolution displays of for very small text or for text that must be read from a great distance.
However, until WWII, the simplification of the letter forms has been pushed too far, resulting in many letters that are too similar, so they can no longer be distinguished. There are many such sans serif typefaces that have been modified very little from their pre-WWII ancestors, like Helvetica and Arial, which are too simplified so that they should be avoided in any computing applications due to the great probability of misreading anything that is not plain English.
After WWII, and especially after 1990, there has been a reversal in the evolution of the sans serif typefaces, away from excessive simplification and towards making them more similar to serif typefaces, except for the serifs.
A well-known example of such a sans-serif typeface is FF Meta (Erik Spiekermann, 1991), but it had a lot of imitators. Such non-simplified sans-serifs typefaces have e.g. traditional Caroline shapes for the lower-case "a", "g" and "l" and also true italic variants (i.e. not just oblique variants).
Besides the removal of serifs, a traditional simplification in the sans serif typefaces is the removal of the contrast between thin lines and thick lines, making the thickness of the lines uniform. At least for me, any long text printed with a font with uniform line thickness looks boring, so I strongly prefer the sans serif fonts that go even further in their resemblance with serif fonts, by having thin lines and thick lines, for example Optima nova and Palatino Sans.
While any sans serif typeface by definition does not have serifs, when Hermann Zapf has designed Optima (which was released in 1958), he has found an alternative to serifs, which achieves a similar optical effect. Starting from a line that has the form of a long rectangle, instead of attaching serifs to the ends, one can make the 2 lateral long edges of the rectangle concave, instead of flat. After that, one has 2 alternatives for how to terminate the ends of the line. The first is to also make concave the 2 short terminal edges. This results in sharp corners for the line and it is the solution chosen by Zapf in Optima. The second method of line termination is to keep the terminal edges flat or even slightly convex and to round the corners where they meet the concave lateral edges. This is the method chosen by Akira Kobayashi in Palatino Sans (2006), under the influence of the similar line terminations used in Cooper Black and in the rounded sans-serif typefaces that are popular for public signage in Japan.
This alternative to serifs, with concave lateral edges, is in my opinion superior both to serifs and to classic sans serifs, but unfortunately it is effective only on very high-resolution displays or on paper (even a cheap laser printer has better resolution than the most expensive monitors), because on low resolution monitors any slightly concave edges will become straight.
In any case, for the best readability, I never use fonts with ambiguous characters, like Helvetica/Arial and most other sans serifs. Instead of that, serif fonts are better, but even better are good modern sans serifs which have been designed carefully, to have distinctive characters. With good monitors, fonts with contrast between thin lines and thick lines, or even with concave lateral line edges, are preferable.
I read this HN thread rendered in the Palatino Sans mentioned in TFA (with the italic of Palatino nova configured as its italic form; the italic of Palatino Sans Informal is also a good choice, but I prefer a stronger contrast between the regular and the italic variants of a font; that is why I have also configured the italic of Bauer Bodoni as the italic for Optima nova).
While in TFA Palatino Sans was dismissed with regret, for having to be purchased, I have bought Palatino Sans, together with a few other high-quality typefaces and I use them on Linux, instead of the available free fonts. I consider the money that I have spent on good typefaces as some of the best purchasing decisions that I have made.
However, for CLI/TUI applications and program editing, I use the free JetBrains Mono. Unlike for regular text, for programming there are many high-quality free fonts, but I prefer JetBrains Mono because it supports an extended character set, with many Unicode mathematical symbols that are missing in other programming fonts.