Take the biotech industry for example. There are also plenty of acquisition and consolidation, but even the biggest top 5 biotech companies combined cannot match the size of one Apple or Microsoft. This is mostly because biotech is so much riskier and a failed product is extremely punishing. At any stage a drug can fail. Even after commercialization and release, a drug can be removed for plenty of reasons. One drug can cost hundreds of millions of dollars to get to market and years and years of R&D by hundreds of scientists going from discovery to production, validation, scaling up, and QC, and marketing. None of the steps can be skipped because the FDA breathes down their neck in every step. People will also sue the company to death if some fuckup happens.
Meanwhile, Microsoft can churn out a new software or product in a year from a team of 20 engineers. And if a fatal error happen, no biggies. Patch incoming and that is that. Lawsuits from bad codes aren't as severe as lawsuits from bad drugs. Leaks and exploits and all the "huge" issues get solved and the product continue to stay on market. A drug fuckup and it is very likely to be scrapped along with all the investments that went into it.
And so big tech continue to grow unchecked because they are just that: unchecked.