There is a difference between budget and more expensive instruments. Though that difference is sometimes exaggerated, it's not completely made up.
The difference tends to fall into two categories :
1. Attention to Detail / Quality
More expensive instruments tend to have more attention to detail as well as higher quality control (correlated). It's just common sense. If you're paying someone more for an instrument you're incentivizing them to make it a higher quality instrument. There's more revenue to pay someone or multiple persons to do the job right as well as to use the best materials (however you define that). Though you can shave off costs by having a high quality instrument made overseas where labor is cheaper there is still a point where you have to pay more to get more quality.
Quality also tends to follow the "law of diminishing returns". That is, the more you pay the less you get back relatively. If you pay 500 dollars you might get 80% "quality", for 1000 USD you might get 90%, for 1500 USD you might get 95%. Those numbers are made-up, but I'm doing so to make a point. The more you pay the more you get, but for every price bracket you move upwards you get less in return. Each person's tolerance and/or preference for quality vs. cost is different.
Thankfully this aspect of basses can often be fixed to some extent if you take your bass to a luthier or technician. You can get a setup, some finish clean-up, and a fret job and make a budget instrument nearly as playable as something much more expensive. The question is, is it worth it? That's up to the individual. Some say no, some say yes, but who are we to force our opinion on others? There are lots of valid reasons why someone might feel either way.
For example, maybe someone only had 300 bucks one day and needed a bass. They deal with it's issues until they can save up some extra money. Then they take it to a tech and resolve any issues it had. They broke their bass purchase up into a payment program essentially, which is what they could afford.
2. Features
Short of some part-swaps to a complete rebuild you won't get a Squier to be exactly like a MIA Fender feature-wise. The MIA Fender has posiflex rods, custom shop pickups, lightweight tuners, dual action truss rods, string through bridge, etc. Certainly you can put a new neck on your Squier or something, but then you just put together a parts bass. It isn't the same bass anymore.
Feature differences are the hardest to fix. Yeah, you could drill your Squier for a string through bridge, replace the neck, throw a new bridge/tuners on it, and put some custom shop pickups in it. However you just spent quite a lot of money doing that. In some cases it's better to just go buy a more expensive instrument. You can get more for the money in some cases, as for example, manufacturers can utilize "Economies of scale" to get the same parts for much cheaper than you can get them a la carte.
On the other hand, some people are habitual modders and they might be better served by a cheaper instrument. If you're going to mod your instruments anyway, why pay for a bunch of features you'll just be tossing out later?