It is not true that "dead", or quenched galaxies cannot start forming stars again. It is called rejuvenation and is observed occasionally (e.g. Chauke et al. 2019), although not commonly (Akhshik et al. 2021). It is also seen in simulations, where some quiescent galaxies return to the star-forming main sequence (Behroozi et al. 2019).
What causes galaxies to cease forming stars is not entirely settled, but is generally thought to be a lack of cold gas. In fact, this was convincingly confirmed only last week by Whitaker et al. (2021) who, using Hubble+ALMA observations and aided by gravitational lensing, showed that six quiescent galaxies in the early Universe lacked virtually all cold gas.
How quenched galaxies have lost their cold gas is another open question. Major mergers was thought to be an efficient way to strip galaxies of their gas, but is probably not the most significant mechanism (Gabor et al. 2010; Weigel et al. 2017). So probably feedback from star formation and/or active galactic nuclei is (at least part of) the reason, either by mechanically blowing out the gas, or by heating it (e.g. Cheung et al. 2016). In principle, the gas should cool within the order of 1 giga-year, but at least for a while it can be kept hot through energy injection from central supermassive black holes (Whitaker et al. 2013) or perhaps gravitational heating (Johansson et al. 2009).
So, rejuvenating through merging is not impossible, although it might just result in a larger quiescent galaxy, as least in the case of "dry" merging, i.e. with another gas-poor galaxy. Minor merging has been suggested as a mechanism to rejuvenate quenched galaxies (Manelli et al. 2014), as has more smooth gas re-fueling (Marino et al. 2011; Mancini et al. 2019).