A couple minor points not addressed above.
First, your tires provide a significant amount of suspension. I don't know how many riders inflate their tires to the maximum pressure on the sidewall. However, this will be a lot less comfortable and it is also likely to be slower. People who study rolling resistance have shown that on real-world surfaces, there is an optimum pressure for each tire that minimizes rolling resistance, and it's lower than many people think (on extremely smooth tracks like track cyclists use, higher pressure does indeed always equal lower rolling resistance). People on paved surfaces don't need suspension. For that matter, cyclists take rigid bikes (i.e. no suspension) onto dirt roads and light gravel all the time.
Second, suspension forks are expensive, and they require maintenance, which is expensive in either your time or your money. At a fixed price point, having a suspension fork detracts from other components. Most newer riders don't realize this, and are essentially demanding capabilities that they don't need. In fact, if taken to the extreme, the suspension will fail entirely, leaving you with dead weight on the frame (although I guess at least your fork won't bob when pedaling).
Third, suspension forks may come with lockouts, which reduce energy-sapping bobbing behavior on the road. However, lockouts tend to be present only on pricier forks.
Fourth, to a point that @AndyP raised: gravel suspension forks are a niche market, and those systems are indeed designed to damp small amplitude bumps rather than for big hits. However, these are rare, and tend to be costlier.
Fifth, if you do want to damp vibrations for comfort, there are suspension seatposts and stems available. The couple of designs that I'm aware of (which is not all of them!) don't require maintenance. However, their pivot points and elastomers will eventually wear out. The elastomers will be replaceable if the originating company is still running and if it kept stock. The pivot points tend not to be repairable. So, while I don't know the potential lifespan of these items, it should be long but not infinite. The ones I'm aware of are relatively expensive designs (e.g. Redshift's stem and seatpost, Cane Creek's seatpost), and cheaper stems and seatposts might not last as long.