Buying a pre-built campervan or commercial campervan ("Class B") would seem to be a real win. In practice, though, there are many advantages of building your own.
Especially after the surge in interest in RV and Van travel that began in 2020, the quality of pre-made vans and RVs has dropped precipitously. It's gotten to the point that even RV dealers themselves are fed up. The amount of money that you'll pay for something of extremely questionable quality is astounding, with top of the line Class-B RVs pushing over $200,000.
Even if you already own your van and take it to an upfitter, you can expect between $25,000-$50,000 (on top of what you paid for the van!) to have a fairly complete build out.
That being said, going full DIY isn't for everyone and Craiglist (or other used vehicle marketplaces) are often full of vans that have been gutted and partially finished by someone who got themselves in over their head without realizing how big the project is. Building out a van is a very significant investment in time, tools and materials. However you can do it for as cheap or as expensive as you want. It all depends on your needs and what you prioritize.
Fortunately there does exist a happy medium of “DIY Conversion Kits” available from companies like RB Components, Paradigm Vans or Adventure Wagon that sell prefabricated parts that bolt into place. This can significantly reduce the amount of time/effort required for a build, but still gives you plenty of space to customize and make it your own. (Note: There are dozens of these type of kits and conversions available, a simple google search will yield a plethora of results targeting all types of builds and skill levels)