External lenses may be more versatile when taking photos on a smartphone, but despite this, many people still don't use them. From cumbersome designs to mediocre results, there are plenty of reasons why they aren't for everyone.
1. Smartphones now have better zoom capabilities than ever before
Smartphone cameras have much better zoom capabilities than they used to, and the cameras are usually fine when using this feature. You can also move in closer instead of zooming for most photos, so there's no need for an external lens. For anything further away, use a mirrorless camera.

Tips: Before buying an external lens, think about what you really want from your phone camera. Built-in optical zoom is often a better solution than fiddling with add-ons.
2. Built-in focus adjustments
Besides image quality, versatility is one of the biggest reasons people buy lenses for their mirrorless cameras. Versatility is going to be a priority with any smartphone lens, but unlike a professional camera, you already have built-in options to adjust focus.
For example, with a light touch, iPhone allows you to toggle between 0.5 and 1 x on the main camera screen. The Photos app also allows cropping parts of the photo that may not appear in the main photo (16:9 is the default, so you don't necessarily see everything when you take the photo).
3. You can add filters to your smartphone
Another reason people buy smartphone camera lenses is to get new filters. There are several ND filters; in many cases, you must clip them to your external lens. But while these filters can help you get more unique results, they're not necessary.
You can add multiple filters to your smartphone photos without downloading third-party apps. Some favorite iPhone camera filters include Vivid and Dramatic Warm. There are also some black and white options.

If you want more, you can download a camera app to take better photos.
4. External lenses are uncomfortable
Some external lenses, especially larger lenses like 50mm zoom lenses, are uncomfortable. They are not budget-friendly and you need to remove them before storing your phone.
Above all, many people want to avoid attracting unnecessary attention when taking photos in public. Keeping things simple will make you enjoy taking photos more.
5. External lenses may cause image distortion
If you've ever taken photos with any lens, you've probably encountered distortion at least once. This happens even with the best mirrorless lenses that cost thousands of dollars. However, this situation occurs more often when using external lenses for smartphones.
Because many smartphone external camera lenses are from third parties, image distortion is worse than when taking photos with the original camera. These problems can be easily fixed with tools like Lightroom, but you may have difficulty without third-party editing software.
These distortions aren't always a big deal, but they are something to consider if you're thinking about buying an external lens.
6. Many people don't like the lens setup
Many people prefer to take street photos with smartphones rather than DSLR cameras, because it is much easier to carry a phone. All you have to do is take your smartphone out of your pocket, open the Camera app and start taking photos. Sometimes, choosing the right settings can be a hassle, even if you don't change lenses on your mirrorless camera.
If you use an external lens instead, you'll have to spend more time setting up your phone. Smaller lenses simply clip on, but you may have to do a more complicated setup for larger lenses. Since phones are much smaller, you may also need to set up a tripod.
The extra setup time is not worth it. Use a mirrorless camera for more complex shots and a smartphone for more casual shots.
7. The app provides the flexibility you need
One argument against smartphone cameras is that they are not as versatile as professional cameras. You can zoom in, but it can't be compared to a professional camera lens. In most cases, you can't even change shutter speed and ISO.

Agreed that smartphones are more limited than dedicated cameras, but you can customize settings more than you think. Like other camera lenses, you can change the aperture in some cases (for example, Apple's Portrait Mode).
If you need more flexibility, you can easily download third-party apps. For example, the ProCamera iOS app has good low-light performance and anti-shake technology.
8. No one wants to carry extra equipment
Adding lenses is already a hassle, and no one wants to carry more equipment than necessary. Having to juggle multiple lenses in your bag is frustrating, and constantly worrying about them takes the fun out of photography.

Another problem is that it makes it easier to forget your device. Having your smartphone camera ready – and knowing you won't miss anything important at home – gives you time to take more photos. This helps you improve your skills naturally.
Perhaps it would be better not to have a camera lens on a smartphone for many reasons. No one likes to carry more than necessary, and taking photos is easier without needing to set up external equipment. In many cases, the zoom function on smartphones is reasonable enough to take beautiful photos.