At lower zoom levels, if all the detail was shown the
screen would be too cluttered. The same is true with paper
charts. The small scale
charts don't show all the detail you need. So you would need to carry many charts to cover the same area with different amounts of detail. Part of responsible navigating to paying attention to that.
OpenCPN had provisions for adjusting what is displayed at different "zoom" levels, but it depends on the specific charts used and is one of the more complicated things to learn. For instance, you can change the chart scale (and thus what is displayed) independent of the zoom level. Some chart types (CM93) have a simple "detail level" setting that you can adjust up/down until you see what you need, and others (ENC) you can select from dozens of chart symbols to display or not. But even if you select to display a symbol type, if the chart scale you are viewing doesn't include that detail, you won't see it.
Navionics sucks. The apparent inability to adjust detail level is only one of many shortcomings. I know there are many popular alternatives in addition to
OpenCPN, and I am sure others will chime in.