Institutions can hire people to become specialists in every industry. They may not have the local knowledge you have in your industry, and there may be a lag before they know things, but they have at least a basic knowledge of every industry. But if you're a retail investor who works in accounting for a dog food manufacturer, it's more difficult to really be able to understand a biotech stock.
Fees
There are fees for everything. You pay a fee when you buy a stock and when you sell it. You pay a higher fee if you do it on margin or if you buy options. You pay annual fees and expenses for any fund you buy. This is even true if you use an app that has no commissions. The way those apps make money is by increasing the bid/ask spread, meaning you pay more for the stock through them than you would through a traditional broker.
Time
Professional investors have the luxury of spending their entire workday analyzing stocks and investing. Retail investors may have to find time to do proper analysis in between lunch and picking kids up from day care.
Access
Large institutions have access to some transactions that aren't available to the public. This could be a Private investment in Public Equity (PIPE), an investment in an initial public offering (IPO), or even an investment in a private company.
Economies of scale
Sometimes the problem of size (as discussed in the liquidity section) is a good thing, at least for institutional investors. When other institutions or even corporations want to buy or sell a huge block of shares, they will often offer a discount or premium to do it all at once. Institutions that can handle that level of transaction can take advantage, while retail investors would always have to pay the market price.
