I'm nowhere close to being bilingual, and there are without doubt many factors involved in this, but I can think of a few fairly easily.

Chinese has relatively few possible syllable sounds compared to Western languages. There are about 400 possible initial-final combinations in Mandarin, and 4 tones they can be said in (5 if you include the neutral tone that can only appear at the end of a word), but not all combinations exist, and most estimates place it at about 1200 for Mandarin Chinese. This compares to about 15,000 odd for English, as a syllable has more flexibility in terms of initials, vowels and finals, and English does in fact have tones - even though we don't think of the tone as being syntactically significant, it is very hard to interpret someone if they deliberately change the pitch in unusual ways. But anyway, Chinese therefore has about 10% of the syllable range of English, but each syllable is in fact a "meaning unit" in its own right, whereas English will frequently use multiple syllables to express one concept, which makes it more redundant / less ambiguous. BTW, I say "meaning unit" because in older dialects of Chinese, each syllable was exactly a word and this was possible because there was more information in a syllable and so it was easier to disambiguate, but at some point things became confusing and Chinese began using pairs of "meaning units" to represent concepts - for instance "speech" in Mandarin is 说话 "shuo"+"hua" where the first word was an older verb for speaking and the second was an older noun for speech. Modern Chinese will still tend to revert back to the simpler forms if it's unambiguous and various grammar forms show that they still think of the words as being separate even if they are usually used together, e.g. in the positive+negative question form, 你喜不喜欢 were 喜欢 is normally considered a single word.

As a learner with a vocabulary of maybe 5000 words (compared to a native with maybe 10k+), I've already encountered a lot of homonyms and sometimes when you're watching a drama it's easier to look at the Chinese subtitles than trying to guess which word they meant. If you were having an actual conversation with someone, you could figure it out from context or ask for clarification, but that doesn't really work for one-way communication. One example: 这个合同是gong1zheng3的 "This contract is 'gongzheng'". Does this gongzheng mean 公正 (fair or equitable) or 公证 (notarised)? Just from speech alone, it's impossible to tell without further clarification or rewording, as both would be perfectly plausible.

In mainland China, there are a lot of people for whom Mandarin isn't their first language. Most people will speak it to varying levels of ability, sure, but regional dialects often have completely different words, pronunciations or even just different tones to Mandarin. In all those cases, being able to read along while listening helps comprehension. I'm not sure about Taiwan, but I'm sure there's a reasonable number of people who primarily speak Hokkien and only use Mandarin when they have to interact with people outside their community / town.

Finally, you also assume it's a choice... In most cases the subtitles are baked into the original broadcast (for TV). Back in the days of analogue TV when closed captions came along fairly late and required an expensive box, and so were only purchased by deaf people, subtitling on the broadcast was an easy way to ensure that everybody could get them, much as foreign TV shown in the West almost always has hard-coded subtitles. So for many people, it might not actually be an active choice - they might just only have sources that have hard-coded subtitles. I find it interesting that platforms where the content is intended for consumption by native Chinese speakers, e.g. TV dramas on youtube, the Chinese subs are usually hard-coded, but when they are sold to foreign platforms they usually don't have them. Personally, I find it quite frustrating that e.g. Viki doesn't often have Chinese subtitles and I want to know what exact phrase was just said, as then I usually have to find the show elsewhere e.g.on youtube.

There might also be the cases where the subtitles could be turned off, but they just don't bother. This might seem strange, I know I hate English subtitles on English shows, but many Chinese will watch TV shows with so many scrolling comments on top of the screen (often called "bullet text") that it's almost impossible to see what's underneath. Some of my Chinese friends actually watch most shows twice - once for the show, and once for all the comments. They probably aren't in the slightest bit concerned about the single line of subtitles at the bottom, especially if it makes watching a little bit easier.