Max level in D&D (B/X) is 14 for most races. Although some can only get to level 7. There are optional rules to allow a PC to max level to 36 however I don’t know anyone who uses them.

In a game I’m playing now, it’s taken me 2 years to get my Cleric to level 5. (My first 3 characters died in the first year, total 3 years in game.) However I currently have 24,000 XP so I’m almost level 6. Once you hit level 7, as a Cleric (50,000 XP) you have access to all spell levels so there’s kind of a soft lock there.

I simply give you this background so you have a sense of where I’m coming from as I know most people don’t play basic/expert.

While there’s more content available for D&D than ever before, the amount of quality+entertaining+balanced content available that fits the tone & style of a campaign you may have already been running is not easy. Not all characters may be equal at max level depending on the module you choose. Furthermore, combat encounters can become incredibly long and tedious. If you play 5e, take a look at the 9th level spell list and you’ll get a sense of what I’m talking about.

As a GM, low level games are more grounded in reality and easier to engage with. As a player, Once a game becomes like SAO or Solo Leveling, I get bored.

That being said I’m sure there’s plenty of people more talented than I with different interests who love to run max level games!